Chlamydia trachomatis is the etiological agent of the most commonly reported bacterial sexual transmitted infection (STI) in North America and Europe. The control of Chlamydia infection is hindered by the asymptomatic nature of initial infection but the consequence of untreated infection seriously threatens the reproductive health of young women. Unfortunately, there is no licensed vaccine for Chlamydia vaccine, in part due to our incomplete understanding of the immune response to Chlamydia urogenital infection. It has been well established that T cell-mediated immunity plays a dominant role in protective immunity against Chlamydia and thus the importance of B cells is somewhat underappreciated. Here, we summarize recent progress on understanding the role of B cells during Chlamydia genital tract infections and discuss how B cells and humoral immunity make an effective contribution to host defense against important intracellular pathogens, including Chlamydia.

B cells can contribute to acquired immunity against intracellular bacteria, but do not usually participate in primary clearance. Here, we examined the endogenous CD4 T cell response to genital infection with Chlamydia muridarum using MHC class-II tetramers. Chlamydia-specific CD4 T cells expanded rapidly and persisted as a stable memory pool for several months after infection. While most lymph node Chlamydia-specific CD4 T cells expressed T-bet, a small percentage co-expressed Foxp3, and RORγt-expressing T cells were enriched within the reproductive tract. Local Chlamydia-specific CD4 T cell priming was markedly reduced in mice lacking B cells, and bacteria were able to disseminate to the peritoneal cavity, initiating a cellular infiltrate and ascites. However, bacterial dissemination also coincided with elevated systemic Chlamydia-specific CD4 T cell responses and resolution of primary infection. Together, these data reveal heterogeneity in pathogen-specific CD4 T cell responses within the genital tract and an unexpected requirement for B cells in regulating local T cell activation and bacterial dissemination during genital infection.

Salmonella infection profoundly affects host erythroid development, but the mechanisms responsible for this effect remain poorly understood. We monitored the impact of Salmonella infection on erythroid development and found that systemic infection induced anemia, splenomegaly, elevated erythropoietin (EPO) levels, and extramedullary erythropoiesis in a process independent of Salmonella pathogenicity island 2 (SPI2) or flagellin. The circulating EPO level was also constitutively higher in mice lacking the expression of signal-regulatory protein α (SIRPα). The expression level of EPO mRNA was elevated in the kidney and liver but not increased in the spleens of infected mice despite the presence of extramedullary erythropoiesis in this tissue. In contrast to data from a previous report, mice lacking EPO receptor (EPOR) expression on nonerythroid cells (EPOR rescued) had bacterial loads similar to those of wild-type mice following Salmonella infection. Indeed, treatment to reduce splenic erythroblasts and mature red blood cells correlated with elevated bacterial burdens, implying that extramedullary erythropoiesis benefits the host. Together, these findings emphasize the profound effect of Salmonella infection on erythroid development and suggest that the modulation of erythroid development has both positive and negative consequences for host immunity.

The obligate intracellular bacterium Chlamydia trachomatis causes the most prevalent bacterial sexually transmitted infection worldwide. CD4 T cells play a central role in the protective immunity against Chlamydia female reproductive tract (FRT) infection, while B cells are thought to be dispensable for resolution of primary Chlamydia infection in mouse models. We recently reported an unexpected requirement of B cells in local Chlamydia-specific CD4 T-cell priming and bacterial containment within the FRT. Here, we sought to tackle the precise effector function of B cells during Chlamydia primary infection. Using mixed bone marrow chimeras that lack B-cell-dependent Ag presentation (MHCII^B-/- ) or devoid of circulating antibodies (AID^-/- × μS^-/- ), we show that Chlamydia-specific CD4 T-cell expansion does not rely on Ag presentation by B cells. Importantly, we demonstrate that antibody, but not B-cell-dependent Ag presentation, is required for preventing systemic bacterial dissemination following Chlamydia FRT infection.

Immune mechanisms responsible for pathogen clearance from the female reproductive tract (FRT) are incompletely defined; in particular, the contribution of lymphocyte trafficking to this process is unclear. CCR7-deficient mice have profoundly altered lymphocyte recirculation and display ectopic formation of lymphocyte aggregates within mucosal nonlymphoid tissues, including the FRT. In this study, we investigated how altered lymphocyte distribution in CCR7-deficient mice would affect host responses to Chlamydia muridarum within the reproductive tract. As expected, CCR7-deficient mice exhibited reduced lymphocyte trafficking to lymph nodes and a corresponding increase in T cell populations within the FRT. After intravaginal infection with Chlamydia, CCR7-deficient mice displayed markedly reduced Ag-specific CD4 T cell responses within the local draining iliac lymph nodes, yet robust Th1 and Th17 responses were prominent in the FRT. In addition, Chlamydia-specific Ab responses were dysregulated in CCR7-deficient mice, displaying an unexpected increase in the systemic IgA responses. Importantly, prominent mucosal immune responses in CCR7-deficient mice increased the efficiency of bacteria clearance from the FRT while reducing tissue-associated inflammation and pathology. Thus, increased numbers of lymphocytes within the FRT result in pathogen clearance with reduced immune-mediated pathology.

Efforts to develop vaccines that can elicit mucosal immune responses in the female genital tract against sexually transmitted infections have been hampered by an inability to measure immune responses in these tissues. The differential expression of adhesion molecules is known to confer site-dependent homing of circulating effector T cells to mucosal tissues. Specific homing molecules have been defined that can be measured in blood as surrogate markers of local immunity (e.g. α4β7 for gut). Here we analyzed the expression pattern of adhesion molecules by circulating effector T cells following mucosal infection of the female genital tract in mice and during a symptomatic episode of vaginosis in women. While CCR2, CCR5, CXCR6 and CD11c were preferentially expressed in a mouse model of Chlamydia infection, only CCR5 and CD11c were clearly expressed by effector T cells during bacterial vaginosis in women. Other homing molecules previously suggested as required for homing to the genital mucosa such as α4β1 and α4β7 were also differentially expressed in these patients. However, CD11c expression, an integrin chain rarely analyzed in the context of T cell immunity, was the most consistently elevated in all activated effector CD8+ T cell subsets analyzed. This molecule was also induced after systemic infection in mice, suggesting that CD11c is not exclusive of genital tract infection. Still, its increase in response to genital tract disorders may represent a novel surrogate marker of mucosal immunity in women, and warrants further exploration for diagnostic and therapeutic purposes.

Toll-like receptors (TLRs) can recognize microbial patterns and utilize adaptor molecules, such as-MyD88 or (TRIF TIR-domain-containing adapter-inducing interferon-β), to initiate downstream signaling that ultimately affects the initiation of adaptive immunity. In addition to this inflammatory role, TLR5 expression on dendritic cells can favor antigen presentation of flagellin peptides and thus increase the sensitivity of flagellin-specific T-cell responses in vitro and in vivo. Here, we examined the role of alternative signaling pathways that might regulate flagellin antigen presentation in addition to MyD88. These studies suggest a requirement for spleen tyrosine kinase, a noncanonical TLR-signaling adaptor molecule, and its downstream molecule CARD9 in regulating the sensitivity of flagellin-specific CD4(+) T-cell responses in vitro and in vivo. Thus, a previously unappreciated signaling pathway plays an important role in regulating the dominance of flagellin-specific T-cell responses.

CD11c is an α integrin classically employed to define myeloid dendritic cells. Although there is little information about CD11c expression on human T cells, mouse models have shown an association of CD11c expression with functionally relevant T cell subsets. In the context of genital tract infection, we have previously observed increased expression of CD11c in circulating T cells from mice and women. Microarray analyses of activated effector T cells expressing CD11c derived from naïve mice demonstrated enrichment for natural killer (NK) associated genes. Here we find that murine CD11c+ T cells analyzed by flow cytometry display markers associated with non-conventional T cell subsets, including γδ T cells and invariant natural killer T (iNKT) cells. However, in women, only γδ T cells and CD8+ T cells were enriched within the CD11c fraction of blood and cervical tissue. These CD11c+ cells were highly activated and had greater interferon (IFN)-γ secretory capacity than CD11c- T cells. Furthermore, circulating CD11c+ T cells were associated with the expression of multiple adhesion molecules in women, suggesting that these cells have high tissue homing potential. These data suggest that CD11c expression distinguishes a population of circulating T cells during bacterial infection with innate capacity and mucosal homing potential.

T cell effector functions can be elicited by noncognate stimuli, but the mechanism and contribution of this pathway to the resolution of intracellular macrophage infections have not been defined. Here, we show that CD4(+) T helper 1 (Th1) cells could be rapidly stimulated by microbe-associated molecular patterns during active infection with Salmonella or Chlamydia. Further, maximal stimulation of Th1 cells by lipopolysaccharide (LPS) did not require T-cell-intrinsic expression of toll-like receptor 4 (TLR4), interleukin-1 receptor (IL-1R), or interferon-γ receptor (IFN-γR) but instead required IL-18R, IL-33R, and adaptor protein MyD88. Innate stimulation of Th1 cells also required host expression of TLR4 and inflammasome components that together increased serum concentrations of IL-18. Finally, the elimination of noncognate Th1 cell stimulation hindered the resolution of primary Salmonella infection. Thus, the in vivo bactericidal capacity of Th1 cells is regulated by the response to noncognate stimuli elicited by multiple innate immune receptors.