Targeting molecular pathways to control immune checkpoint inhibitor toxicities

Abstract

Immune checkpoint inhibitors (ICIs) have transformed cancer treatment but are frequently associated with immune-related adverse events (irAEs). This article offers a novel synthesis of findings from both preclinical and clinical studies, focusing on the molecular mechanisms driving irAEs across diverse organ systems. It examines key immune cells, such as T cell subsets and myeloid cells, which are instrumental in irAE pathogenesis, alongside an in-depth analysis of cytokine signaling [interleukin (IL)-6, IL-17, IL-4), interferon γ (IFN-γ), IL-1β, tumor necrosis factor α (TNF-α)], integrin-mediated interactions [integrin subunits αITGA)4 and ITGB7], and microbiome-related factors that contribute to irAE pathology. This exploration of modifiable pathways uncovers new opportunities to mitigate irAEs by using available antibodies (Abs) that target key inflammatory molecules across tumor types, while ideally preserving the antitumor efficacy of ICIs.

ICI-induced toxicities

irAEs (see Glossary) have emerged as significant challenges in the era of immune checkpoint inhibitor (ICI) treatment for cancer. While ICIs have revolutionized cancer therapy by harnessing the human body’s immune system to destroy tumor cells, they can also induce a spectrum of immunological side effects. These toxicities can affect any organ system, including the skin, gastrointestinal (GI) tract, liver, and endocrine glands [1]. Management of irAEs typically requires treatment with immunosuppressive medications. Systemic corticosteroids, such as prednisone or methylprednisolone, are frequently used as first-line therapy for moderate to severe irAEs [2]. Guidelines for irAEs recommend initiating treatment with 1–2 mg/kg of prednisone for most irAEs of grade 3 or higher [2]. They work by suppressing lymphocytes and other immune cells globally, which may negatively affect ICI efficacy [3]. Initially, high doses may be prescribed, followed by a taper once symptoms improve to minimize negative effects on antitumor immunity. However, steroids appear to significantly hinder ICI efficacy, particularly in the early phases of treatment [3]. In cases of refractory or severe irAEs, additional immunosuppression may be necessary, including treatment with agents that target specific pathways involved in the inflammatory response, such as Janus kinase (JAK) inhibitors [4]. Depending on the severity of the irAE, temporary or permanent discontinuation of ICIs may be required [1]. For example, myocarditis is a particularly severe irAE that can be fatal [5]. A meta-analysis indicated that using a single ICI or a combination of ICI and chemotherapy could significantly increase the risk of ensuing cardiotoxic events compared with chemotherapy alone [6]. Treatment might be delayed until the irAE resolves or is adequately controlled. Understanding the immunopathology underlying irAEs is crucial for managing these adverse events effectively and ensuring the continuity of lifesaving cancer immunotherapies. Therefore, this review explores the immunological targets involved in irAEs, organized into subtopics focused on various immune cells (such as T and B cells and myeloid cells) as well as the influence of the host microbiome on these adverse events. Based on diverse inflammatory pathways, potential drug targets are being investigated. The ultimate goal is to develop targeted therapies that not only mitigate irAEs but also enhance the therapeutic efficacy of ICIs, paving the way for more personalized and safer immunotherapy approaches.

T cell-targeted irAE-therapies

The mammalian skin and GI tract serve as frontline barriers against pathogens and play pivotal roles in immunosurveillance and regulation. ICI can lead to diverse cutaneous irAEs, ranging from mild rashes to severe blistering disorders and life-threatening reactions such as toxic epidermal necrolysis [7]. Identifying the specific molecular and cellular targets involved in irAEs is essential for elucidating their underlying mechanisms and developing targeted interventions. Recent studies using technologies such as single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics have identified expanded populations of CD8⁺ and CD4⁺ tissue-resident memory T cells (T_RM cells) during GI and cutaneous irAEs in patients with diverse tumors such as renal cancer, lung cancer, or melanoma [4,7]. These cells exhibit a type 1 T helper (Th1)/cytotoxic T cell (Tc1)-polarized phenotype and are observed in conditions such as immune-related (ir)-dermatitis and ir-colitis in humans, as well as in ir-myocarditis in mice [4,7,8]. These T_RM cells exhibit high expression of relevant inhibitory checkpoint molecules such as PD-1, CTLA-4, TIGIT, TIM3, and LAG3 [9]; indeed, checkpoint blockade may reactivate these cells locally within target tissues. A strong IFN-γ signature – with expression of IFN-γ-induced genes such as HLA-DRs and CD74 – has been reported in the tissue of cutaneous and GI irAEs from patients treated with an anti-PD-1 Ab alone or in combination with anti-CTLA-4 Ab (ICIs) [7]. Additionally, transcripts of downstream chemokines such as CXCL9, CXCL10, and CXCL11 were upregulated in the same tissue samples [7]. These CXCR3 ligands potentially attract additional circulatory T cells to inflamed irAE tissue because they bind to their cognate receptor CXCR3 on the surface of T cells [9–11]. In vitro trans-well migration assays demonstrated that CXCR3 ligands such as CXCL9 and CXCL10 were each independently sufficient to recruit human CD8⁺ effector T cells [10]. A recent study utilizing flow cytometry and proteomics demonstrated the expansion of Ki-67⁺ T cells subsets in blood and the early increase in CXCL9/CXCL10/CXCL11 and IFN-γ (1–2 weeks after the start of ICI) in sera from patients with melanoma and lung cancer receiving anti-PD-1 Ab ± anti-CTLA-4 Ab (ICIs). These outcomes preceded the development of irAEs, including the development of colitis, pneumonitis, dermatitis, and/or hepatitis [12].

Together, these data suggest that Th1/Tc1 subsets contribute to triggering irAEs through cytokine secretion, indicating that immunosuppressive strategies targeting Th1/Tc1 signatures might be beneficial in irAE management. One available option might be the topical application of JAK inhibitors on skin to combat irAEs [13]; presumably, this might target Th1/Tc1 cell subsets better than corticosteroids because they would prevent IFN-γ signaling, and would thus stop downstream inflammatory pathways in the skin (Box 1).

Box 1. Topical JAK inhibition in cutaneous irAEs.

A first case report described successful treatment of corticosteroid-refractory eczematous ir-dermatitis with the topical JAK1/2 inhibitor ruxolitinib (1.5% cream) in a patient with breast cancer receiving pembrolizumab (anti-PD-1 Ab) [13]. Topical JAK inhibition might represent a possibility for targeting localized cutaneous irAEs without the risk of interfering with antitumor immunity. Systemic JAK inhibition, such as tofacitinib (blocking JAK1/3), has been used in refractory ir-colitis mediated by IFN-γ-producing T_RM cells in a patient with anti-PD-1 Ab-associated ir-colitis [4]. However, systemic JAK inhibition might also abrogate immunotherapy efficacy by blocking IFN-γ production of antitumor T cells [82].

In addition to IFN-γ-production, increased TNF-α signaling was also observed within the CD4⁺ and CD8⁺ T_RM cell compartment in human tissue samples from cutaneous and GI irAEs in patients with cancers such as melanoma and lung cancer, compared with healthy tissues [4,7]. These findings suggest that TNF-α blockade might be a viable therapeutic strategy for managing irAEs associated with human tumors. TNF-α blockade – achieved through medications such as infliximab, adalimumab, and etanercept – is commonly used to treat various autoimmune and inflammatory conditions, including rheumatoid arthritis, psoriasis, and inflammatory bowel disease [14]. In particular, infliximab was successful in targeting ir-colitis but has also been used for other irAEs such as pneumonitis, acute interstitial nephritis, myocarditis, and myositis [15]. In a preclinical model, prophylactic TNF-α blockade reduced irAEs (specifically colitis and hepatitis) in immunodeficient Rag2^−/−Il2rg^−/− mice xenografted with human colon cancer cells, while preserving the therapeutic efficacy of dual checkpoint blockade [16]. A melanoma mouse model lacking TNF (Tnf^–/– mice) demonstrated increased numbers of tumor-infiltrating CD8⁺ T lymphocytes and reduced tumor growth relative to that in controls [17]. This suggested that TNF signaling via TNFR1 could hinder the accumulation of CD8⁺ T cells within the tumor microenvironment (TME), thereby promoting melanoma progression [17]. In a KPC mouse model of subcutaneous pancreatic adenocarcinoma (PDAC), blocking TNFR1 (using Tnfr1^−/− mice) increased the number of intratumoral dendritic cells and enhanced antitumor immunity compared with controls (CD8⁺ T cell proliferation and effector function) [18]. In addition, TNF can mediate activation-induced cell death (AICD) in vitro in human tumor-reactive CD8⁺ T cells derived from various cancer patients undergoing surgery; this effect was mediated via TNFR2 signaling and the production of mitochondrial reactive oxygen species (ROS) leading to DNA damage [19]. The use of TNF-blocking agents such as infliximab and etanercept reversed these effects, suggesting that these drugs might be helpful for enhancing immunotherapy strategies, although this requires further investigation [19]. In another study, TNF-α and IL-1β induced IL-8 expression in human cancer cell lines (colon, lung, renal, and breast cancers, and melanoma), promoting an immunosuppressive TME by activating immunosuppressive tumor-infiltrating myeloid leukocytes [20].

However, contradictory results exist that suggest that TNF-α can cause tumor cell necrosis and thus may be important for immune-mediated tumor regression in some settings [21]. A Phase 1b clinical study assessed the safety and efficacy of combining TNF blockers (infliximab or certolizumab) with nivolumab and ipilimumab ICIs in advanced melanoma patients (small cohort: n = 2–3 per group). The authors reported that both combinations seemed to be safe with encouraging antitumor responses, as evidenced from objective response rates (ORRs) and increased systemic numbers of Th1 cells and total T cells; the primary endpoint was occurrence during the first 12 weeks of dose-limiting toxicities (DLTs), while the secondary endpoints were ORRs and progression free survival (PFS) (NCT03293784ⁱ) [22]. The certolizumab cohort showed a particularly high response rate, with complete and partial responses, suggesting potential for further investigation [22].

Apart from cytokines IFN-γ and TNF-α, other interesting molecular targets related to T cell trafficking and homing exist. In patients with various tumors treated with anti-PD-1 ± anti-CTLA-4 Ab ICIs who developed cutaneous and GI irAEs, high expression of the ITGA4 gene has been reported [7]. Blocking ITGA4 with natalizumab (anti-ITGA4 Ab) or vedolizumab (anti-ITGA4 + ITGB7 Abs) demonstrated efficacy for ICI-meningoencephalomyelitis and steroid-refractory ir-colitis in patients with various tumors such as melanoma or renal or prostate cancers [23,24]. Vedolizumab, a monoclonal Ab targeting α4β7 integrin, is primarily utilized for the treatment of Crohn’s disease and ulcerative colitis/irritable bowel diseases (IBDs) [25]. It has also shown efficacy for steroid- or infliximab-refractory GI irAEs [26]. Treating ir-colitis patients with either infliximab or vedolizumab resulted in a better ICI response than treating patients only with steroids [26]. This result underscores the beneficial impact of achieving a more precise immunomodulating approach. Dynamic adhesion and transmigration assays on T cells from microscopic colitis patients showed that α4β7 integrin led to binding adhesion molecule MAdCAM-1, supporting the transmigration of CD4⁺ T cells [27]. Importantly, inhibiting the function of these integrins with vedolizumab prevented transmigration of CD4⁺ T cells in vitro [27]. Vedolizumab’s mechanism of action, specifically targeting gut-specific lymphocyte trafficking, makes it a promising candidate for managing GI irAEs.

IL-17, primarily produced by type 17 helper (Th17) cells, represents another key player in the pathogenesis of irAEs. Elevated serum IL-17 concentrations were found to correlate with the subsequent occurrence of severe ir-colitis in neoadjuvant ICI-treatment of melanoma patients [28]. Dysregulated IL-17 signaling was also implicated in Th17-driven cutaneous irAEs, such as psoriasis-like eruptions, and in case reports of patients with melanoma and colon cancer that were treated with anti-PD-1 Abs [29,30]. Accordingly, therapeutic targeting of IL-17 pathways via neutralizing Abs has demonstrated efficacy in ameliorating cutaneous psoriasiform irAEs [29,30]. Specifically, IL-17 promoted keratinocyte proliferation and activation of signal transducer and activator of transcription 3 (STAT3), contributing to the production of CCL20 and inflammation in skin tissues of psoriasis patients [31]. Moreover, in a study of ir-thyroiditis, the number of IL-17A-producing innate-like γδT (γδT17) cells was increased in tumor-free mice following ICI treatment (anti-PD-1 ± anti-CTLA-4 Abs); by contrast, Th17 cells were prominent in C57/BL6 tumor-bearing mice (MC38 tumor cells) [24]. In this mouse model of ir-thyroiditis, IL17A-blockade combined with ICI not only treated the irAE but also maintained immunotherapy efficacy [32]. However, in a case report involving a patient with metastatic colon cancer and ir-psoriasis, IL-17A-blockade resulted in the resolution of psoriatic lesions but also in the loss of antitumor responses (tumor growth) to anti-PD-1 Ab pembrolizumab, and was detrimental to long-term survival [29].

In cases of bullous cutaneous eruptions, IL5 transcripts were detected by RNA in situ hybridization in archived tissue from two patients who developed cutaneous irAEs during anti-PD-1 Ab therapy for melanoma, suggesting potential Th2 cell involvement [7]. In various case reports of patients with either eczematous or bullous cutaneous irAEs, Th2 cells were targeted directly via dupilumab, an IL-4 receptor α antagonist Ab [33–35], which blocks IL-4 and IL-13 signaling [34]. Response to anti-CTLA-4 ± anti-PD-1/PD-L1 Abs seemed to be unaffected in these patients [34], suggesting that targeting the Th2 axis might be a promising avenue for certain irAE treatments, pending rigorous investigations. Of note, a Phase 1b/2 clinical trial explored IL-4 signaling blockade (dupilumab) together with anti-PD-1 Ab ICI, and suggested that this approach may harbor potential for improving clinical responses in patients with relapsed or refractory metastatic non-small-cell lung cancer (NSCLC) (primary endpoint: DLT and ORR) (NCT05013450ⁱⁱ) [36]. Another biologic, benralizumab (anti-IL-5 Ab), was used as a biomarker to phenotypically profile IL-5 in the plasma and skin of 12 patients with ICI-induced maculopapular rash (MPR). The authors concluded that this type of biomarker analysis could be helpful in determining irAE endotypes that require precision medicine approaches for treatment [37]. We posit that a personalized treatment approach for irAEs should be considered, rather than a global immunosuppression approach (i.e., treatment with corticosteroids). To achieve this, it will be important to classify the inflammatory patterns and cells involved in irAEs. For instance, in cutaneous irAEs, determining the polarization of circulatory and skin T cells (Th1 versus Th2 versus Th17) or T_RM cells may be helpful for identifying individual putative therapeutic targets [37].

Myeloid cell-targeted irAE therapies

TNF-α is not only produced by T cells in irAE immunopathologies, it can also be secreted by myeloid cells. In the GI tissues of melanoma patients with ir-colitis (compared with healthy patients), TNF signaling was significantly upregulated in macrophages, as evidenced by scRNA-seq [38]. The same study showed that myeloid cells overexpressed IL-1β in ir-colitis GI tissues [38]. Moreover, in a study of melanoma patients treated with PD-1 ICIs (alone or with anti-CTLA-4 Ab), researchers profiled blood cytokines; the expression of these correlated with the severity of irAEs [39]. Indeed, circulating IL-1β was identified as one of the key cytokines that might serve as a predictive marker for severe irAEs [39]. Other work showed that IL-1β was upregulated in macrophages derived from patients with PD-1 ICI-induced ir-arthritis relative to controls [40]. This study utilized scRNA-seq to analyze blood and synovial fluid, demonstrating that IL-1β-high myeloid cells attracted circulating T cells via chemokines such as CXCL10 and CCL5 [40]. It has been thoroughly discussed that CXCL10 and CCL5 can recruit effector T cells to the TME of melanoma patients treated with anti-PD-1 ± anti-CTLA-4 Abs [11]. These findings were corroborated in a study describing myeloid cells expressing CXCL9, CXCL10, CXCL11, and CXCL16 in patients bearing tumors such as melanoma or renal or lung cancers that exhibited ICI-induced ir-arthritis [41]. Chemokine production prompted CD8⁺ T cell influx into inflamed joints, as evidenced from scRNA-seq and T cell receptor (TCR) sequencing (TCR-seq) on blood and synovial fluid samples [41]. Similarly, myeloid cells in patients exhibiting ir-colitis showed high levels of expression of CXCR9–11, contributing to the influx of effector T cells into inflamed GI tissues [38]. Of note, IL-1β-high myeloid cells can be targeted via IL-1β blocking agents such as anakinra (IL-1 receptor antagonist Ab) or canakinumab (anti-IL-1β Ab) [42]. These medications have been clinically used to treat various autoimmune diseases such as Still’s, Behcet’s, and Schnitzler diseases [42]. Experimental evidence from a renal-cell carcinoma mouse model suggests that IL-1β blockade can also promote tumor regression by decreasing the number of tumor-infiltrating myeloid-derived suppressor cells (MDSCs) and increasing the number of so-called M1-like tumor-associated macrophages [43]. Several years ago, researchers proposed a treatment schedule of either anakinra 100 mg once per day, or canakinumab 300–600 mg once every 8 weeks for severe or refractory irAEs such as colitis or arthritis in patients with anti-PD-1 ± anti-CTLA-4 Ab-treated tumors (in general) [44]. However, recent work indicated that adding canakinumab to anti-PD-1 Ab therapy plus chemotherapy did not enhance antitumor efficacy or provide additional clinical benefit in PFS or overall survival (OS) in patients with NSCLC [45,46]. Therefore, treatment regimens remain to be carefully assessed.

Of note, elevated serum IL-1α concentrations have been detected in patients with advanced GI cancers and ir-myositis [47]. Accordingly, in a C57BL/6 mouse model of B16.F10 melanoma tumors, blocking IL-1α signaling significantly enhanced the efficacy of ICIs against tumors by promoting the depletion of MDSCs [48]. This is interesting because inhibiting both IL-1α and IL-1β might potentially help abrogate irAEs and perhaps synergize with ICIs, which will warrant rigorous assessment. Presumably it may be more beneficial to test anakinra instead of canakinumab in this context, because anakinra blocks the activity of both cytokines [49].

From another angle, targeting the contribution of myeloid cells to irAE occurrence might be achieved by blocking the IL-6 pathway. Indeed, dysregulated IL-6 signaling can lead to severe and potentially lethal inflammation [50]. Intracellular signaling is initiated when IL-6, IL-6R, and gp130 assemble into a heterohexameric complex, which then recruits cellular signaling proteins such as JAK and STAT3 [51]. The IL-6/JAK/STAT3 pathway is upregulated in many cancers, and potentially in irAEs [51]. Tocilizumab, an anti-IL-6R Ab, has emerged as a promising strategy for managing irAEs [53,54], particularly for patients experiencing rheumatological irAEs (e.g., ir-arthritis) in various types of malignancies such as melanoma, genitourinary cancer, or lung cancer [54]. Accordingly, retrospective data have indicated that prophylactic tocilizumab can have clinical benefit in ICI-treated melanoma patients by preventing irAEs while preserving ICI efficacy (anti-PD-1 ± anti-LAG3 or anti-CTLA-4 Abs) [53]. This finding has been confirmed prospectively in a Phase 2 study. Here, the addition of prophylactic tocilizumab to ipilimumab and nivolumab therapy in patients with metastatic melanoma reduced severe irAEs without compromising efficacy [56] (Box 2). In addition, elevated concentrations of serum IL-6 have been associated with various irAEs, including ir-dermatitis in patients with liver cancer [55]. Further investigation into these cytokine- and chemokine-associated myeloid cell-targeted therapies should ideally lead to promising outcomes for treating irAEs in malignancies.

Box 2. Targeting IL-6R to mitigate ICI-induced toxicity.

Dysregulated IL-6 contributes to off-target inflammation by promoting Th17 cell differentiation, as well as neutrophil recruitment and tissue damage in ir-colitis presented by melanoma patients treated with anti-PD-1 and/or anti-CTLA Abs [40,71]. IL-6R blockade reduces the frequency of macrophages, MDSCs, and Th17 cells in the tumor tissues of mice bearing B16. BL6 melanomas, as measured by mass cytometry [71]. Combination ICI (anti-CTLA-4 and anti-PD-1 Abs) has been shown to increase the number of Th17 cells in ir-arthritis compared with anti-PD-1 Ab monotherapy, as demonstrated by an scRNA-seq study analyzing blood and synovial fluid [41]. In tumor-bearing mice with autoimmune encephalomyelitis (EAE), IL-6R blockade together with ICI mitigated EAE and enhanced the effect of anti-CTLA-4 Ab therapy. Also, 31 cancer patients treated with tocilizumab or sarilumab for irAEs related to anti-PD-1 therapy exhibited a significantly better overall response following IL-6R blockade as well as resolution of irAEs relative to controls [71].

B cell-targeted irAE therapies

While B cells can impact immunotherapy responses by contributing to T cell activation [57–61], they can contribute to off-target toxicity during ICI therapy via Ab production. A systematic review reported that auto-Abs were detected in nearly 50% of patients with various ICI-treated (anti-PD-1 Ab and/or anti-CTLA-4 Ab) cancers (mainly melanoma, NSCLC, renal/urothelial cancers), and with the patients experiencing ICI-induced endocrinopathies [62]. In patients with melanoma treated with anti-PD-1 Ab therapy, the presence of auto-Abs, especially anti-thyroid Abs, predicted to some extent irAEs such as thyroid dysfunction [63]. Notably, these auto-Abs did not correlate with melanoma recurrence or treatment response, indicating that they might specifically serve as markers for toxicity rather than efficacy. In patients with NSCLC receiving anti-PD-1/PD-L1 Ab therapy, elevated baseline anti-BP180 IgG titers (measured via ELISA), significantly correlated with the development of cutaneous irAEs [64]. Similarly, cancer patients (i.e., patients with melanoma or NSCLC) with myositis/myasthenia/myocarditis often presented with Abs such as striational Abs (49%), acetylcholine receptor Abs (40%), and myositis-associated Abs (27%), suggesting that auto-Abs might play a role in the development of some irAEs [62]; 30% of patients with sicca symptoms associated with ICI therapy harbored antiphospholipid Abs (Sjögren’s syndrome) [62]. Conversely, only 11% of arthritis patients tested positive for either rheumatoid factor, or anticyclic citrullinated peptide (CCP) Abs [62]. Auto-Abs were also less common among patients with hepatitis (antinuclear or ANA Abs, 18%) or colitis (perinuclear antineutrophil cytoplasmic or p-ANCA Abs, 19%) [62]. Taken together, B cells producing auto-Abs might be involved in the occurrence of some irAE endotypes. Related to this, rituximab, an Ab targeting CD20 on B cells, might be considered as a putative approach to ameliorate ICI-induced autoimmune disorders that include an auto-Ab profile, although this warrants robust testing. Of note, as part of a larger study, one patient with ir-bullous pemphigoid was successfully treated with rituximab [34] (Table 1). However, the potential negative impact of rituximab administration on the efficacy of systemic therapy (e.g., ICI) remains a concern. A study examining pretreatment B cell numbers in the peripheral blood of anti-PD-1 Ab-treated cancer patients (various types of cancers) reported that higher B cell numbers were significantly associated with improved treatment responses, which was encouraging [65]. Not surprisingly, B cells are important within the TME and can reside in tertiary lymphoid structures together with follicular T helper cells (Tfh cells); these immune cell hubs have been reported to contribute to antitumor immune responses in mice and in human cancer patients across tumor types [57,58,60] (Figure 1, Key figure). Nevertheless, there are also positive reports on rituximab therapy for some patients. For instance, a patient receiving targeted therapy – BRAF/MEK inhibition for metastatic melanoma, and rituximab for rheumatoid arthritis – achieved durable complete remission despite experiencing B cell depletion due to the drug treatment [66]. Other cases of anti-PD-1 Ab-treated melanoma patients receiving rituximab and experiencing tumor shrinkage have been reported [67]. It is hypothesized that rituximab may enhance anti-PD-1 Ab efficacy by targeting regulatory B cells that produce immunosuppressive IL-10 [67]. This concept was partly supported by a study in μMT mice, which showed that splenic B cells continuously produced IL-10 and contributed to an immunosuppressive environment [68].

Table 1.

Examples of current therapeutic irAE-targets in humans and mice^a

Species, number of patients[Refs]	Type of irAE	Molecular targets	irAE treatment	Cell compartment	Potential effect on ICI	Cancer type	Cancer treatment (e.g., ICI)
22 patients [7]	Dermatitis (most: lichenoid or MPR) Dermatitis (two cases BP)	Local IFN TNF IL-5	Proposed topical JAKi	Th1/Tc1, TRM cells (Th2 cells)	–	Melanoma	Anti-PD-1 ± anti-CTLA-4 Abs
One patient [13]	Dermatitis (eczematous)	Local IFN	Topical JAK1/2i Ruxolitinib	Th1 cells	Neutral	Breast cancer	Chemotherapy and anti-PD-1 Ab
Mice and one patient [4]	Colitis (steroid/infliximab/FMT refractory colitis)	Syst IFN	Systemic JAKi Tofacitinib	Tc1 cells	Negative	NSCLC	Chemotherapy + anti-PD-L1 Ab
185 patients [15]	Colitis, nephritis, myocarditis, pneumonitis, myositis	TNF	Infliximab	T cells/myeloid cells	Neutral	Melanoma, genitourinary	Anti-PD-1 ± anti-CTLA-4 Abs
Mice [32]	Thyroiditis	IL-17A	Anti-IL-17A Ab	Th17 cells	Neutral	Melanoma Colon cancer	Anti-PD-1 Ab
One patient [30]	Dermatitis (psoriasiform)	IL-17A	Secukinumab	Th17 cells	Neutral	Melanoma	Anti-PD-1 Ab
One patient [29]	Dermatitis (psoriasiform)	IL-17A	Secukinumab	Th17 cells	Negative	Colon cancer	Anti-PD-1 Ab after chemotherapy
Seven patients [34]	Dermatitis: BP Eczematous Psoriasiform Psoriasiform	CD20 IL-4 IL-23 IL-12/23	Rituximab Dupilumab Guselkumab Ustekinumab	B cells Th2 cells Th17 cells	Neutral	Melanoma Renal-cell carcinoma Lymphoma Lymphoma	Anti-PD-1 ± anti-CTLA-4 Abs Anti-PD-1 ± anti-CTLA-4 Abs Chemotherapy + anti-PD-L1 Ab Chemotherapy + anti-PD-L1 Ab
Three patients [35]	Dermatitis: eczematous, BP	IL-4	Dupilumab	Th2 cells	Neutral	cSCC Melanoma Serous endometrial carcinoma	Anti-PD-1 Ab ± Lenvatinib
One patient [33]	Dermatitis: BP	IL-4	Dupilumab	Th2 cells	Neutral	Melanoma	Anti-PD-1 Ab
92 patients [54]	Mainly rheumatological irAEs, minority with myositis/myocarditis/myasthenia gravis, pneumonitis, nephritis, colitis, scleroderma, CNS vasculitis	IL-6	Tocilizumab	Neutr. Th17 cells	Neutral	Melanoma, genitourinary cancer, and lung cancer	Anti-PD-1 ± anti-CTLA-4 Abs
Mice, 31 patients [71]	Arthritis (77%); remainder: systemic sclerosis, myositis/myocarditis/myasthenia, hepatitis, CNS vasculitis, encephalitis	IL-6	Tocilizumab + sarilumab	Neutr. Th17 cells	Positive	Melanoma	Anti-PD-1 ± anti-CTLA-4 Abs

^aAbbreviations: Ab, antibody; BP, bullous pemphigoid; CNS, central nervous system; cSCC, cutaneous squamous-cell carcinoma; I, inhibitor; FMT, fecal microbiota transplantation; JAKi, Janus kinase inhibitor; MPR, maculopapular rash; Neutr., neutrophils.

Key figure Figure 1. Cellular and molecular targets involved in immune checkpoint inhibitor (ICI)-related toxicity.

The figure also depicts the targets that might be therapeutically perturbed with clinically available antibodies. Antibodies can be directed toward various cell compartments based on the predominant cell types involved in immune-related adverse events (irAEs). For instance, antibodies can target specific T cell subsets, such as Th1, Th17, and Th2 cells, as well as myeloid cell subsets such as macrophages [13,32,34,71]. Antibodies can also target autoantibody-producing B cells [34]. Integrin-blocking antibodies can disrupt the adhesion of circulating immune cells, potentially leading to inflammation associated with irAEs [24,26]. Spheres represent secreted chemokines/cytokines. Abbreviations: Eos, eosinophil; IFN, interferon; IL, interleukin; ITGA4, integrin α4; ITGB7, integrin β7; JAKi, Janus kinase inhibitor; Mac, macrophage; Neutro, neutrophil; Th, T helper cell; TNF-α, tumor necrosis factor α; top., topical. Figure created with Biorender.

In addition, a stem-cell-like subpopulation of melanoma cells that has been associated with a worse prognosis of patients with melanoma expresses CD20, and might potentially be targeted with rituximab [67,69] (Box 3). Nevertheless, extensive and robust experiments are required to (i) further investigate the potential of using rituximab for different combination treatments in cancer patients, and (ii) obtain a deeper understanding of putative strategies for B cell-targeted irAE therapies.

Box 3. The relationship between the microbiome and irAEs.

Another area of investigation is evaluating the role of the commensal microbiota in irAEs in barrier organs such as the skin or gut. Studies with C57BL/6 and Foxp3-DTR mice bearing B16F10 melanoma tumors with CTLA-4-induced colitis showed that administration of Bifidobacterium sp. resulted in significantly less weight loss without compromising the therapeutic efficacy of ICI relative to controls [83]. In a mouse model of ICI treatment (anti-CTLA-4 Ab), the commensal bacterium Staphylococcus epidermidis was associated with ir-dermatitis, where most bacteria-specific T cell subsets produced IFN-γ or IL-17 [52]. The majority of these T cells were identified as T_RM cells which expanded in response to CTLA-4 blockade in skin tissue [52]. However, further research is needed to validate these targets and to rigorously explore the role of commensal microbiota in irAEs within barrier organs such as these.

Concluding remarks

With increasing approvals for ICI use in a variety of cancer types, irAEs have become a frequently encountered medical issue. IrAEs can necessitate treatment pauses or discontinuation, which can be critical for metastatic cancer patients. Systemic glucocorticosteroids are the standard of care and are often effective, yet global immunosuppression may diminish immunotherapeutic efficacy. Thus, achieving more targeted immunosuppression may be advantageous. Th1/Tc1 circulatory or tissue-resident memory T cells are crucial for the ICI antitumor response, particularly in the neoadjuvant setting, because they can mediate responses to ICI via IFN-γ signaling [9,70]. A recent report of a patient with breast cancer suggested that blocking irAE-related IFN-γ-producing T_RM cells in cutaneous irAEs might be locally effective by using JAK inhibitors, while preserving the systemic antitumor immune response [13]. As previously discussed, IL-6R blockade might enhance ICI efficacy while also mitigating irAEs [71]. Moreover, this dual positive effect of IL-6R blockade suggests that it may harbor potential for prophylactic application [53]. However, conflicting results have been reported for IL-17, a cytokine that is downstream of IL-6. Indeed, a recent study indicated that high baseline IL-17 concentrations in the tumor and blood predicted a better clinical response to dual-ICI treatment (anti-PD-1 + anti-CTLA-4 Abs) in BRAFV600-mutated melanoma patients [72]. Therefore, blocking IL-17 to treat irAEs might diminish the effects of ICIs and, if implemented, should be employed with caution.

Targeting macrophage-derived cytokines (such as TNF-α, IL-1β, or IL-6), in addition to T cells, could potentially enhance the efficacy of ICIs and alleviate irAE symptoms [73]. It might help to reverse the immunosuppressive phenotype of macrophages, as demonstrated in vivo via scRNA-seq in B16F10 melanoma tumor-bearing C57BL/6J mice [73]. Of note, blocking IL-6R with tocilizumab and IL-1R with anakinra have been effective strategies for managing severe cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS), respectively [74,75]. Indeed, these adverse events are commonly associated with therapies involving bispecific T cell engaging receptor (TCER) or chimeric antigen receptor (CAR) T cells [74,75].

Of note, a recent study of PD-1 ICI-induced mouse ir-hepatitis exemplified the importance of targeting other chemokine/cytokine pathways to curtail irAEs; the use of Rag2^−/− and Ccr2^rfp/rfp transgenic mice (along with flow cytometry, immunofluorescence, and RNA-seq) showed that CCR2 deficiency significantly reduced CCR2⁺ monocytes in the liver [76]. This reduction led to fewer cytotoxic CD8⁺ T cells being recruited to the liver, which effectively prevented the progression of experimental ir-hepatitis [76].

From another angle, blocking IL-4Rα impacts not only T cells (i.e., Th2 cells) but also directly influences the myeloid cell compartment; in humans and mice, research has shown that IL-4 signaling favors the induction of immunosuppressive myeloid cell states within the TME [36]. IL-4 that is derived from basophils and eosinophils can indeed enhance the development of immunosuppressive, tumor-promoting myeloid cells [36]. Building on this understanding, a small Phase 1b/2 clinical trial tested a combination of anti-PD-1/PD-L1 Ab and an anti-IL-4Rα blocking Ab in refractory or relapsed NSCLC patients that had progressed through anti-PD-1/PD-L1 Ab therapy alone (see discussion earlier) (NCT05013450ⁱⁱ) [36]. This combination therapy resulted in activation of the Th1 immune response axis (IFN-γ and IL-12), and upregulated expression of IFN-γ-induced chemokines CXCL9–11. Treatment with dupilumab led to CD8⁺ T cells expansion, as well as dendritic cell and B cell activation, as evidenced from cytometry by time-of-flight (CyTOF) on whole blood; this outcome was linked to a partial response in one of six anti-PD-1/PD-L1 Ab refractory patients with NSCLC [36]. Given these initial findings, it is plausible to suggest that, depending on disease and context, blocking IL-4Rα might help achieve two objectives: enhancing the clinical efficacy of ICI against tumors while effectively treating certain irAEs.

Across various cancers and types of irAEs, consistent evidence indicates that the occurrence of irAEs often correlates with a better response to ICI [77–79]. This underscores the challenge of treating irAEs without adversely affecting ICI efficacy. Germline genetic variants, such as rs7036417, have been associated with severe irAEs in patients with melanoma independently of ICI efficacy, highlighting the need for further research into the relationship between germline single nucleotide polymorphisms (SNPs) and the occurrence of irAEs [80]. Across various cancer types, such as melanoma, breast cancer, or GI cancer, a recent analysis in multi-organ irAE patients demonstrated that ocular irAEs often co-occurred with cutaneous and GI irAEs, while rheumatological irAEs frequently co-occurred with musculoskeletal irAEs [79]. In the future, it will be of interest to distinguish between organ-specific subtypes of irAEs at a molecular level. In particular, cutaneous irAEs should be evaluated and documented for their varying inflammatory patterns (see Outstanding questions). It seems that bullous and eczematous irAEs are rather Th2-oriented, psoriasiform eruptions are more Th17-driven, and the vast majority of lichenoid and maculopapular rashes are triggered by Th1/Tc1 T_RM cells [7,30,34], findings that should be further evaluated. Of note, there is growing concern about long-lasting toxicities that can impact various organ systems such as endocrine, rheumatological, GI, dermatological, neurological, and cardiovascular [81]. Indeed, chronic irAEs present significant challenges for long-term cancer survivors. Effective management of chronic irAEs requires a multidisciplinary approach between oncologists, rheumatologists, dermatologists, and immunologists, and researchers continue to elucidate the efficacy, optimal dosing, and long-term safety profile of these agents in the context of ICI therapy. We argue that demonstrating clinical benefit from a targeted therapeutic approach for irAEs should involve multiple strategies; sequential biopsies from both affected tissues and blood samples, taken at the onset of irAE and following targeted therapy initiation, can provide insight into the treatment’s impact on irAEs, assessing in translational studies the therapeutic effects on irAE underlying mechanisms (Figure 2). Additionally, randomized clinical trials that compare targeted approaches to standard treatments will be essential for determining whether targeted interventions can increase treatment success and reduce the time to irAE resolution.

Outstanding questions.

How can we accurately assess the molecular and cellular patterns of irAE-affected tissues when tissue biopsies are less accessible, for example in cases other than ir-dermatitis?

Is it feasible to reliably subclassify cutaneous irAEs into distinct inflammatory patterns based on T cell polarization (Th1, Th2, Th17, etc.)? This would be helpful in the development of targeted treatment strategies for each specific irAE subtype/endotype.

What methods can be employed to predict severe irAEs early, allowing for timely adjustments to ICI regimens?

Could targeted interventions, such as cytokine blockade, be administered prophylactically to prevent the occurrence of irAEs and potentially enhance the efficacy of ICIs?

What genetic factors contribute to the susceptibility and occurrence of irAEs?

Figure 2. Example of a translational study design to evaluate the efficacy of targeted therapies for immune checkpoint inhibitor (ICI)-induced toxicities.

Step 1: collect blood and tissue samples at the onset of immune-related adverse events (irAEs). Step 2: conduct multiomics analyses to identify the predominant inflammatory patterns. Step 3: select targeted therapy based on the individual’s specific inflammation profile. Step 4: monitor the clinical response of ICI-induced toxicity to the targeted therapy, and evaluate post-treatment response in blood and tissue by analyzing follow-up biopsies. Figure created with Biorender.

Highlights.

Understanding the molecular mechanisms underlying immune-related adverse events (irAEs) stemming from immune checkpoint inhibitor (ICI) therapies, and phenotyping each cancer patient based on individual inflammatory patterns, are vital to achieve personalized treatments.

T and B lymphocytes, as well as myeloid cell subsets, play important roles in irAE development.

Interventions that target molecules/mediators associated with these cell types, such as cytokines or integrins, offer promising strategies for managing irAEs. These approaches have the potential to mitigate toxicities while maintaining the antitumor efficacy of ICIs.

Certain cytokine-blocking antibodies – such as anti-interleukin (IL)-6R – have the potential to curtail irAEs while also enhancing the efficacy of ICIs for certain malignancies.

Significance.

The approval of immune checkpoint inhibitors (ICIs) for various solid tumors is expanding, reflecting their growing role in cancer treatment. However, the rise in ICI usage is associated with a concomitant increase in immune-related adverse events (irAEs), highlighting the need for clinical management as the therapeutic landscape evolves. The types of induced irAEs are generally similar across tumor types, but the effectiveness of ICI therapies may vary. Immune cellbased therapeutic approaches hold promise across a range of tumor types because irAEs are often tumoragnostic; thus, they merit further mechanistic exploration.

Glossary

Bullous pemphigoid

an autoimmune skin disorder characterized by large, fluid-filled blisters (bullae) on the skin. It primarily affects older adults and arises when the immune system mistakenly attacks the basement membrane of the skin; it involves Abs against BP180 and BP230.

Commensal microbiota

a diverse community of microorganisms, including bacteria, fungi, and viruses, which inhabit various mucosal surfaces and body sites; they play a critical role in maintaining homeostasis by influencing the development and function of the immune system and regulating metabolic processes.

CTLA-4-induced colitis

a type of inflammatory bowel disease presenting as an irAE in patients receiving cancer immunotherapy with CTLA-4 inhibitors, such as ipilimumab. Management typically involves corticosteroids and, in refractory cases, additional immunosuppressive agents such as infliximab or vedolizumab to control inflammation.

CXCR3 ligands (CXCL9/10/11)

signaling chemokines that bind to the CXCR3 receptor, which is found primarily on immune cells such as T cells (CXCL9, CXCL10, and CXCL11); they play a crucial role in directing immune cells to sites of inflammation, infection, or tumors.

Follicular T helper cells (Tfh cells)

a CD4⁺ T cell subset that plays a crucial role in supporting the formation and function of germinal centers within lymphoid follicles; Tfh cells facilitate the activation, maturation, and differentiation of B cells.

ICI-induced endocrinopathies

conditions in which the body’s endocrine (hormone-producing) glands (e.g., thyroid, adrenal, pituitary) are affected by an immune response triggered by ICIs; examples are thyroiditis, hypophysitis, adrenalitis, and diabetes mellitus.

ICI meningoencephalomyelitis

a rare but serious neurological irAE associated with ICI therapy; it involves inflammation of the meninges, brain, and spinal cord (myelitis) due to an immune response triggered by ICIs. It can manifest with a range of symptoms, including headache, confusion, seizures, motor and sensory deficits, and other neurological abnormalities.

Immune checkpoint

a regulatory pathway in the immune system that helps to maintain self-tolerance and modulates the intensity and duration of immune responses. Such checkpoints involve specific proteins on immune cells, such as PD-1, PD-L1, LAG3, TIM3, and CTLA-4, which, when engaged, act as ‘brakes’ to suppress immune activity. They are essential for preventing autoimmunity and controlling inflammation, but cancer cells can exploit them to evade immune detection. Immune checkpoint inhibitors are drugs that block these proteins, thereby ‘releasing the brakes’ on immune cells (i.e., CD8⁺ cytotoxic T cells) to attack cancer cells.

Immune-related adverse events (irAEs)

side effects from ICIs used in cancer therapy that overstimulate the immune system. Overstimulation can potentially lead to inflammation and damage to various organs and tissues, including the skin, liver, lungs, and intestines. Managing irAEs is crucial to balancing effective cancer treatment with minimizing harmful immune reactions.

JAK inhibitors

inhibitors that block the activity of Janus kinases (JAKs), a family of enzymes involved in signaling pathways that regulate immune responses, inflammation, and cell growth. By inhibiting JAK signaling, these drugs reduce the activity of cytokines and other mediators driving inflammatory and autoimmune processes.

Lichenoid and maculopapular rashes

skin reactions that may occur as irAEs during ICI therapy. A lichenoid rash resembles lichen planus and is characterized by small, flat-topped, itchy, purple or reddish bumps. Lesions can affect not only the skin but also mucous membranes. The maculopapular rash features both macules (flat, discolored spots) and papules (small, raised bumps) that may be red and can vary in size.

M1-like tumor-associated macrophages

arbitrarily named proinflammatory macrophages, these are typically activated by signals such as IFN-γ and microbial products [e.g., lipopolysaccharide (LPS)]. Activated M1-like macrophages produce large amounts of proinflammatory TNF-α, IL-1, and IL-6. Can be effective at killing pathogens and tumor cells.

Myeloid-derived suppressor cells (MDSCs)

a diverse population of immature myeloid cells with potent immunosuppressive functions. They originate from the bone marrow and accumulate in various pathologies, including cancer, chronic infections, and inflammatory diseases. Broadly categorized into granulocytic/polymorphonuclear MDSCs (PMN-MDSCs) resembling neutrophils, and monocytic MDSCs (M-MDSCs) resembling monocytes. Because of their immunosuppressive role, MDSCs are targets of interest in developing cancer immunotherapies.

Sicca symptoms

dryness of mucous membranes, commonly affecting the eyes and mouth; characteristic of autoimmune conditions such as Sjögren’s syndrome.

Striational Abs

auto-Abs targeting proteins within striated (striped) muscle fibers, often associated with certain autoimmune neuromuscular disorders such as myasthenia gravis. They can recognize various muscle proteins, including myosin, involved in muscle contraction and structure.

Tertiary lymphoid structures (TLSs)

organized, lymph-node-like structures forming in non-lymphoid tissues; they are composed of immune cells such as T and B cells, dendritic cells, and other components that facilitate a local immune response similar to that seen in lymph nodes. In cancer, TLSs are often associated with better prognosis because they can support an antitumor immune response within the TME.

Type 1 T helper (Th1)/cytotoxic T cell (Tc1)-polarized phenotype

a subset of immune cells – that have differentiated to produce a characteristic set of proinflammatory cytokines, primarily IFN-γ, along with for example, TNF-α. This polarization supports a type 1 immune response which is particularly effective against intracellular pathogens such as viruses, and is involved in antitumor immunity.

Type 17 helper (Th17) cells

a CD4⁺ T cell subset of proinflammatory T helper cells characterized by their production of IL-17 and other related cytokines (such as IL-21 and IL-22); they play a crucial role in defending against extracellular pathogens, particularly fungi and certain bacteria, and are involved in the immune response at mucosal surfaces such as the gut and skin. Th17 cells are also implicated in the pathogenesis of various autoimmune and inflammatory diseases such as psoriasis.

Tissue-resident memory T cells (T_RM cells)

a specialized subset of T cells that persist in peripheral tissues long after an initial infection or inflammation. Unlike circulating memory T cells, tissue-resident memory T cells remain stationed in tissues such as the skin or GI tract, and play a crucial role in maintaining long-term immunosurveillance; they can significantly contribute to tissue-specific immunity.