Abstract
CCR2, a member of the G protein-coupled receptor (GPCR) superfamily, is widely expressed on monocytes, macrophages, activated T cells, and other cell types, and plays a critical role in coordinating the immune response to various infections. Here we demonstrate that CCR2 expression is significantly elevated during Chlamydia muridarum (C. muridarum) respiratory infection, and its absence leads to exacerbated susceptibility, as evidenced by significant weight loss, higher bacterial loads, severe lung pathology, and elevated levels of inflammatory cytokines (il-1β, tnfα, and il-6). The absence of ccr2 impairs both myeloid cell infiltration and T cell responses, which are crucial for effective immune defense. Specifically, ccr2 deficiency disrupts the differentiation and response of Th1 cells, which are the primary effector lineage responsible for clearing chlamydia through secretion of interferon-gamma (IFN-γ). As a result, there is a significant decrease in CD3+CD4+IFN-γ+ T cells in the lung and spleen, accompanied by reduced levels of IFN-γ protein and mRNA, as well as downregulated mRNA expression of Th1-promoting cytokines (il-12p35, il-12p40) and transcription factors (stat4, T-bet), which play crucial roles in Th1 differentiation. Moreover, ccr2 deficiency greatly diminishes STAT1 phosphorylation, a key regulator of IFN-γ secretion by Th1 cells. Meanwhile, we also observed a significant reduction in IFN-γ secretion by CD8+ T cells following ccr2 deficiency. Conversely, ccr2-/- mice exhibit an exaggerated Th2-type immune response, with elevated levels of Th2-promoting cytokines (IL-4), transcription factors (STAT6 and gata3), and il-5, which together lead to more severe lung tissue damage and increased susceptibility to infection. Furthermore, these mice show higher levels of IL-17 along with an enhanced Th17-type immune response, characterized by increased Th17-promoting cytokines TGFB, transcription factors stat3 and RORγt, and il-21, suggesting a compensatory mechanism that drives neutrophil infiltration to exacerbate lung inflammation. These findings underscore the pivotal role of CCR2, a chemokine receptor, in orchestrating the immune response to Chlamydia infection by facilitating Th1 cells differentiation while restraining Th2-type and Th17-type immune responses, thereby alleviating pulmonary inflammation.