Why are immune adverse events so common with Checkpoint Inhibitor Therapy?

Recent advances in the understanding of cancer immunology and T cell immunoreceptor (TCR) signaling facilitated the development of a new class of anticancer drugs - checkpoint inhibitors (CPI) that have been successfully used for cancer treatment. According to 2020 JAMA Network estimate, up to 233,790 US patients with cancer are eligible for CPI. While the CPI blockade of TCR inhibitory signaling pathways can successfully restore/stimulate antitumor immune response, up to 40% of patients develop immune-related adverse events (irAE) during treatment. irAEs have significant impact and may result in dose interruption/discontinuation, negatively affecting clinical outcomes, survival, quality of life, and even lead to mortality. Discontinuation of immunotherapy due to irAEs occurs in up to 73% of patients ¹. Understanding the pathogenesis of irAE is critical for developing new prevention/mitigation strategies. The underlying mechanisms of irAEs remain a significant knowledge gap. Common types of irAEs are reviewed in a companion article by Goleva et. al. published in this issue.

One of the underinvestigated areas in irAEs is the T cell-mediated autoimmune component associated with the loss of immune tolerance to crossreactive autoantigens, in the context of inhibitory immunoreceptor signaling pathways in T cells susceptible to autoimmune activation. Potentially autoreactive circulating T cells that have escaped the central immune tolerance mechanisms during maturation remain in circulation and are vulnerable to autoimmune activation by crossreactive autoantigens.² This autoreactivity is normally kept in check by peripheral immune tolerance checkpoints that rely on modulation/inhibition of TCR signaling via co-receptors (PD-1, CTLA-4) and their ligands. Although T cells with checkpoint-induced peripheral immune tolerance still retain low affinity to the chronically present autoantigens, they are unable to generate immune response due to the intrinsic immunoreceptor signaling inhibition.

It is also not well understood why CPI treatment generates various types of irAEs (cutaneous, pulmonary, thyroid, cardiac, neuromuscular, rheumatological, renal). Characterization of immunoinflammatory mechanisms that preferentially target specific tissues/organs and development of predictive criteria for the type of irAE most likely to develop in each case is another major underinvestigated area.

Link between CPI immunotherapy and the rise of autoimmunity

Available evidence suggests that irAEs in patients treated with CPI may be driven by excessive inhibition of TCR inhibitory phosphatases associated with CPI receptors that maintain peripheral immune tolerance towards autoantigens. This introduces an autoimmune component to the inflammatory reactions in patients with irAEs. Immune checkpoint inhibition (ICI) is a reversible signaling phenomenon which relies on co-receptors providing negative secondary signals to inhibit the “undesirable” T cell responses in order to maintain tolerance to self tissues and prevent autoimmunity. This mechanism can be highjacked by tumor cells overexpressing ligands for ICI receptors (PDL-1/2) to avoid the attack by immune system.³ Animal models of ICI deficiencies demonstrate pronounced autoimmune defects. ICI regulatory proteins are expressed on lymphocytes and many non-lymphoid cells, with expression levels often correlating with immune activation status. Furthermore, cellular responses to CPI engagement often depend on the timing, antigen presentation context and co-receptor signaling.³ It is therefore possible that over-efficient CPI therapy may push the activation of anti-tumor T cell clones further into hyperactivation of autoreactive T cell clones and provoke inflammatory autoimmune response (Fig 1).

Figure 1.

The balance of optimal activation and hyperactivation of T cells with Immune checkpoint inhibitors. Systemic over-depletion of CPI can also disable normal immune checkpoints that maintain self-tolerance of potentially autoreactive T cells. Undesirable activation of self-reactive T cell clones is a byproduct of hyperinhibition of the negative TCR regulatory pathways.

Potential mechanism for CPI-induced hyperactivation of T cells

While CPI-induced activation of T cells enhances anti-tumor immune response, it may also trigger undesirable autoimmune/autoinflammatory side effects in normal tissues. We hypothesize that autoimmune irAEs result from activation-biased TCR signaling induced by over-elimination of inhibitory CPI receptors, which reduces peripheral immune tolerance in potentially autoreactive T cells predisposed to tissue-specific activation (Fig 1). Characterization of this mechanism may identify distinct autoreactive immunoreceptor signaling endotypes associated with specific irAE phenotypes.

Due to the lack of systematic studies, preexisting autoimmune conditions have not yet been recognized as counterindications to CPI therapy⁴, with current guidelines recommending primarily corticosteroid treatments or withholding CPI therapy in advanced grades of irAE. More data is needed to understand associations between immunoreceptor signaling endotypes and clinical phenotypes underlying the diversity of autoimmuninty-related irAEs.

Experimental approaches to identifying potentially autoreactive TCR signaling endotypes

Multiparameter flow cytometry is a well-fitting experimental setting to asses TCR signaling in multiple T cell subsets and detect signaling inhibitory biases by simultaneously measuring phosphorylation/activation levels of major protein kinases and phosphatases involved in TCR signal transduction. This approach facilitates direct assessment of ex vivo TCR signaling responses to anti-CD3/CD28 in patients receiving CPI, with or without irAEs, and on their resolution. Decreased phosphorylation of TCR inhibitory phosphatases (SHP2) and increased phosphorylation of signaling promoters (kinases Zap70, Erk1/2, Pi3K) in patients with irAEs will reveal preexisting TCR signaling biases in specific T cell subsets.

Phosphorylation/activation levels of intracellular signaling proteins can be measured through a combination of intracellular staining with phospho-specific antibodies to signaling proteins and surface CD markers. A similar experimental setting was recently utilized by our group in a separate study to characterize immunoreceptor signaling patterns in patients with autoimmunity and has identified distinct signaling features associated with the loss of peripheral immune tolerance.⁵

This TCR signaling assay may demonstrate that irAEs in CPI-treated patients who are prone to autoimmunity are driven by excessive inhibition of signaling phosphatases downstream of the CPI receptor, as these phosphatases normally inhibit TCR signaling in autoreactive T cells (Fig 1). A measurable diagnostic parameter would rely on a finding that hyperinhibition of the phosphatases is inversely correlated with hyperactivation of the kinases. This signaling imbalance can lead to hyperactivation of potentially autoreactive T cells involved in irAEs. Comparison of TCR signaling profiles in different T cell subsets will help identify circulating cells that contributore to autoimmune inflammatory irAEs.

Better mechanistic understanding of CPI-induced autoimmuninty and other factors that contribute to irAEs will help clarify the management of patients with contraindications to CPI, such as underlying autoimmune conditions, and identify new approaches to mitigate CPI-induced irAEs. Identification of irAE-related immune endotypes will provide rationale for further investigation of balanced immunotherapy approaches based on combined administration of CPI to stimulate anti-tumor cytotoxicity and immunosuppressors targeting divergent autoinflammatory/autoimmune pathways to alleviate irAEs.

Abbreviations/Acronyms:

CPI

checkpoint inhibitors

irAE

immune-related adverse events

TCR

T cell immunoreceptor