Think Locally, Act Globally: Resolving the Peripheral Immune Milieu in Bacterial Pneumonia

Bacterial pneumonia continues to be a major driver of morbidity and mortality (1). Challenges in investigations of pneumonia include the heterogeneity of pathogens, increasing rates of antimicrobial resistance, and the host–pathogen response. These host–pathogen interactions occur not only in the lung—involving tissue-resident immune cells alongside recruited leukocytes—but also in the periphery (2). As a result, the local and systemic manifestations of bacterial pneumonia are not only a result of microbial factors but also due to sequelae of the host response. Modulating this host response through the use of steroids has been beneficial, particularly in severe community-acquired pneumonia (3). However, some patients die of refractory illness, and targeted immunomodulation in pneumonia has not shown consistent benefit. Interrogating individual contributors of the host response by means of an unbiased approach has the potential to facilitate prognostication, identify endotypes, and develop targeted immunomodulatory strategies to mitigate mortality in bacterial pneumonia (4).

In this issue of the Journal, Xiao and colleagues (pp. 2363–2381) generate a single-cell RNA-sequencing (scRNA-seq) atlas of the peripheral immune system in bacterial pneumonia (5). This technology has identified key immune mediators in coronavirus disease (COVID-19) (6), but the present work forms one of the first such atlases in bacterial pneumonia. The authors analyzed scRNA-seq data of peripheral blood mononuclear cells (PBMCs) drawn within 24–72 hours of hospital admission from 100 patients: 39 with severe pneumonia, 31 with mild pneumonia, and 30 from healthy control patients. Severe pneumonia was defined using published criteria (7) and was associated with a dominant proinflammatory signature along the S100A8-TLR4-MYD88 axis in peripheral monocytes. Severe pneumonia was also associated with an exhausted signature in peripheral innate-like T cells and effector CD8⁺ T cells and decreased regulatory T cells, as well as decreased transcripts associated with antigen presentation, B cell–receptor signaling, and costimulation in peripheral B cells. On comparing these findings to those from an earlier study performed by the same group using BAL fluid (BALF) from patients with bacterial pneumonia (Figure 1), the authors identified similar inflammatory pathways in certain myeloid subsets in both compartments (8). Thus, by applying scRNA-seq to PBMCs, the authors discerned granularity in the peripheral immune landscape and identified surrogates for intrapulmonary immune responses in bacterial pneumonia.

Figure 1.

Similarities and differences in peripheral versus local signatures in severe bacterial pneumonia. The existing study identifies the S100A8-TLR4-MYD88 axis from classical CD14⁺ monocytes as a key component of the host immune response in single-cell RNA sequencing (scRNA-seq) conducted on peripheral blood mononuclear cells (PBMCs). Other effects on adaptive immune responses are listed. In comparison, a prior study (8) from the same authors on scRNA-seq from the BAL fluid also identifies the S100A8-TLR4-MYD88 sourced from monocytes and neutrophils as a component of the local immune response in severe bacterial pneumonia. Created with BioRender.

Among the peripheral innate immune responses in patients with severe bacterial pneumonia, the authors highlight the S100A8-TLR4-MYD88 axis as a major driver of inflammation, primarily in several classical monocyte clusters (5). S100A8 is part of the S100 family of proteins that can induce other inflammatory cytokines and can itself serve as a ligand for Toll-like receptor 4 (TLR4) and subsequent signaling through MyD88 (9). Patients with severe pneumonia demonstrate an upregulation of S100A8/9/A12, as well as the dominant expression of TLR4 and MYD88 in these peripheral classical monocyte subsets, suggesting a proinflammatory positive-feedback loop. The authors utilized the top 10 expressed cytokines to develop a score. Three S100 signatures contributed 99% to this score as key inflammatory drivers in severe pneumonia; other elevated cytokines included CXCL8 and TNFSF13. In interpreting these findings, an important consideration is that the predominant etiology of pneumonia in this cohort was Gram-negative pathogens, which also signal through TLR4. It is also interesting that paquinimod, an inhibitor of the S100A8-TLR4-MYD88 axis, has shown benefit in preclinical models of Pseudomonas pneumonia (10). Critical questions raised by the authors’ findings include whether the S100A8-TLR4-MYD88 axis is also relevant to the pathogenesis of pneumonia because of Gram-positive bacteria and viruses, which have distinct pathogenic mechanisms independent of TLR signaling. In this context, a control group with severe viral pneumonia and in vivo modeling would strengthen the observations and provide insights into a therapeutic window and relevant patient populations for targeting this axis.

Beyond the innate immune response, the authors also report that severe bacterial pneumonia is associated with exhausted peripheral effector CD8⁺T cells, defined by the expression of inhibitory receptors PD-1, LAG3, and CTLA-4. Some of these features overlap with those of viral pneumonia. Moreover, peripheral B cells from these patients exhibited decreased expression of genes associated with antigen recognition, B cell–receptor signaling, and costimulation. These observations could be potentially be strengthened using ex vivo functional studies, as has been done with sepsis (11). Additional investigation is important because immune checkpoint inhibitors have been utilized in preclinical models of pneumonia with mixed results (12) and carry the risk of pneumonitis (13). Subsequent studies utilizing preclinical models and longitudinal specimens would clarify whether the exhausted peripheral adaptive immune response represents a protective reaction in severe pneumonia against persistent inflammatory stimuli or whether exhaustion is a maladaptive reaction that should be therapeutically reversed.

A limitation of relying on peripheral immune signatures in lung disease is the question of whether they are truly representative of the local microenvironment. The same group had previously reported scRNA-seq from patient BALF (8), which reveals certain parallels between the peripheral and local immune responses in bacterial pneumonia. For example, patients with severe pneumonia demonstrated increased local S100A8 predominantly sourced from macrophages and neutrophils. Notably, BALF CD8⁺T cells from these patients did not exhibit exhaustion, compared with those from PBMCs in the present study (5, 8). This observation raises the question of whether these differences underly the specific cell types on the basis of their location, time course, clustering and/or annotation, or function. Moreover, another difference between the BALF versus the PBMC scRNA-seq dataset is the detection of neutrophil programs. BALF scRNA-seq profiling facilitated the identification of immature neutrophil populations, which promote excessive inflammation, in part through CXCL8, and suppress T cell activation (8). In contrast, neutrophils were not quantified in this study (5). Future research should involve how best to noninvasively interrogate neutrophil signatures and functions in pneumonia to facilitate targeted interventions in scenarios where neutrophils are driving the underlying pathogenesis (14).

Collectively, Xiao and colleagues provide a valuable single-cell atlas of peripheral immune cells in bacterial pneumonia. Their work identifies a proinflammatory innate immune response and an exhausted adaptive immune response in the periphery associated with disease severity that should be investigated as a prognostic signature in these patients and utilized as a foundation to work toward targeted immunomodulatory interventions. Additionally, these datasets, when made available publicly, offer an opportunity to better understand what is similar across different etiologies of lung injury and what is unique to the pathogenesis of bacterial versus viral pneumonia (15, 16).