TCR Signaling and CD28/CTLA-4 Signaling Cooperatively Modulate T Regulatory Cell Homeostasis.

Foxp3+ T regulatory cells (Tregs), conventional CD4+Foxp3- T cells, and CD8+ T cells represent heterogeneous populations composed of naive phenotype (NP, CD44low) and memory phenotype (MP, CD44high) subpopulations. NP and MP subsets differ in their activation state, contribution to immune function, and capacity to proliferate in vivo. To further understand the factors that contribute to the differential homeostasis of NP/MP subsets, we examined the differential effects of CD28 and CTLA-4 interaction with CD80/CD86, as well as MHC class II-TCR interaction within mouse Treg pools and CD4+ and CD8+ T cell pools. Blockade of CD80/CD86 with CTLA-4-Ig markedly reduced the cycling and absolute numbers of MP Tregs and MP CD4+ T cells, with minimal effect on the NP T cell subpopulations. Blockade of MHC class II-TCR interaction led to selective expansion of MP Tregs and MP CD4+ and CD8+ T cells that was reversed upon cotreatment with CTLA-4-Ig. Treatment with anti-CTLA-4 mAb altered MP Treg and MP CD4+ and CD8+ T cell homeostasis in a manner similar to that observed with anti-MHC class II. We postulate a complex pathway in which CD28 is the primary driver of Treg proliferation and CTLA-4 functions as the main brake but is likely dependent on TCR signals and CD80/CD86. These findings have important implications for the use of biologic agents targeting such pathways to modulate autoimmune and neoplastic disease.

Antibody effector functions mediated by Fcγ-receptors are compromised during persistent viral infection.

T cell dysfunction is well documented during chronic viral infections but little is known about functional abnormalities in humoral immunity. Here we report that mice persistently infected with lymphocytic choriomeningitis virus (LCMV) exhibit a severe defect in Fcγ-receptor (FcγR)-mediated antibody effector functions. Using transgenic mice expressing human CD20, we found that chronic LCMV infection impaired the depletion of B cells with rituximab, an anti-CD20 antibody widely used for the treatment of B cell lymphomas. In addition, FcγR-dependent activation of dendritic cells by agonistic anti-CD40 antibody was compromised in chronically infected mice. These defects were due to viral antigen-antibody complexes and not the chronic infection per se, because FcγR-mediated effector functions were normal in persistently infected mice that lacked LCMV-specific antibodies. Our findings have implications for the therapeutic use of antibodies and suggest that high levels of pre-existing immune complexes could limit the effectiveness of antibody therapy in humans.

Endogenous mouse mammary tumor viruses (mtv): new roles for an old virus in cancer, infection, and immunity.

Mouse Mammary Tumor Viruses are beta-retroviruses that exist in both exogenous (MMTV) and endogenous (Mtv) forms. Exogenous MMTV is transmitted via the milk of lactating animals and is capable of inducing mammary gland tumors later in life. MMTV has provided a number of critical models for studying both viral infection as well as human breast cancer. In addition to the horizontally transmitted MMTV, most inbred mouse strains contain permanently integrated Mtv proviruses within their genome that are remnants of MMTV infection and vertically transmitted. Historically, Mtv have been appreciated for their role in shaping the T cell repertoire during thymic development via negative selection. In addition, more recent work has demonstrated a larger role for Mtv in modulating host immune responses due to its peripheral expression. The influence of Mtv on host response has been observed during experimental murine models of Polyomavirus- and ESb-induced lymphoma as well as Leishmania major and Plasmodium berghei ANKA infection. Decreased susceptibility to bacterial pathogens and virus-induced tumors has been observed among mice lacking all Mtv. We have also demonstrated a role for Mtv Sag in the expansion of regulatory T cells following chronic viral infection. The aim of this review is to summarize the latest research in the field regarding peripheral expression of Mtv with a particular focus on their role and influence on the immune system, infectious disease outcome, and potential involvement in tumor formation.

Regulatory T-cell expansion during chronic viral infection is dependent on endogenous retroviral superantigens.

Regulatory T cells (Treg) play critical roles in the modulation of immune responses to infectious agents. Further understanding of the factors that control Treg activation and expansion in response to pathogens is needed to manipulate Treg function in acute and chronic infections. Here we show that chronic, but not acute, infection of mice with lymphocytic choriomeningitis virus results in a marked expansion of Foxp3(+) Treg that is dependent on retroviral superantigen (sag) genes encoded in the mouse genome. Sag-dependent Treg expansion was MHC class II dependent, CD4 independent, and required dendritic cells. Thus, one unique mechanism by which certain infectious agents evade host immune responses may be mediated by endogenous Sag-dependent activation and expansion of Treg.

TGF-beta-induced Foxp3+ regulatory T cells rescue scurfy mice.

Scurfy mice have a deletion in the forkhead domain of the forkhead transcription factor p3 (Foxp3), fail to develop thymic-derived, naturally occurring Foxp3+ regulatory T cells (nTreg), and develop a fatal lymphoproliferative syndrome with multi-organ inflammation. Transfer of thymic-derived Foxp3+ nTreg into neonatal Scurfy mice prevents the development of disease. Stimulation of conventional CD4+Foxp3(-) via the TCR in the presence of TGF-beta and IL-2 induces the expression of Foxp3 and an anergic/suppressive phenotype. To determine whether the TGF-beta-induced Treg (iTreg) were capable of suppressing disease in the Scurfy mouse, we reconstituted newborn Scurfy mice with polyclonal iTreg. Scurfy mice treated with iTreg do not show any signs of disease and have drastically reduced cell numbers in peripheral lymph nodes and spleen in comparison to untreated Scurfy controls. The iTreg retained their expression of Foxp3 in vivo for 21 days, migrated into the skin, and prevented the development of inflammation in skin, liver and lung. Thus, TGF-beta-differentiated Foxp3+ Treg appear to possess all of the functional properties of thymic-derived nTreg and represent a potent population for the cellular immunotherapy of autoimmune and inflammatory diseases.

Removal of Wolbachia from Brugia pahangi is closely linked to worm death and fecundity but does not result in altered lymphatic lesion formation in Mongolian gerbils (Meriones unguiculatus).

Approximately 30 years ago, researchers reported intracellular bacteria in filarial nematodes. These bacteria are relatives of the arthropod symbiont Wolbachia and occur in many filarial nematodes, including Brugia pahangi and Brugia malayi. Wolbachia bacteria have been implicated in a variety of roles, including filaria development and fecundity and the pathogenesis of lymphatic lesions associated with filarial infections. However, the role of the bacteria in worm biology or filarial disease is still not clear. The present experiments support previous data showing that tetracycline eliminates or reduces Wolbachia bacteria in B. pahangi in vivo. The elimination of Wolbachia was closely linked to a reduction in female fecundity and the viability of both sexes, suggesting that the killing of Wolbachia is detrimental to B. pahangi. The gerbils treated with tetracycline showed reduced levels of interleukin-4 (IL-4) and IL-5 mRNA in renal lymph nodes and spleens compared with the levels in B. pahangi-infected gerbils not treated with tetracycline. However, similar findings were noted in B. pahangi-infected gerbils treated with ivermectin, suggesting that the loss of circulating microfilariae, not the reduction of Wolbachia bacteria, was associated with the altered cytokine profile. Despite the change in T-cell cytokines, there was no difference in the sizes of renal lymph nodes isolated from gerbils in each treatment group. Furthermore, the numbers, sizes, or cellular compositions of granulomas examined in the lymphatics or renal lymph nodes did not differ with treatment. These data suggest that Wolbachia may not play a primary role in the formation of lymphatic lesions in gerbils chronically infected with B. pahangi.

Characterization of antibody responses to Wolbachia surface protein in humans with lymphatic filariasis.

Symbiotic Wolbachia organisms of filarial nematodes have received much attention as possible chemotherapy targets and disease-causing organisms. In order to further investigate the association between anti-Wolbachia immune responses and chronic filarial disease in humans, antibody responses to Wolbachia surface protein (WSP) were assayed in serum samples collected from 232 individuals living in Leogane, Haiti, an area where Wuchereria bancrofti infection is endemic, and from 67 North Americans with no history of lymphatic filariasis. As opposed to antifilarial antibody responses, which were largely influenced by the patient's infection status, the prevalence and levels of anti-WSP immunoglobulin G (IgG) antibodies among individuals with lymphedema or hydrocele were significantly greater than those in gender- and infection-matched individuals without disease. In at least one case, the anti-WSP IgG response was coincident with the onset of lymphedema development, and among anti-WSP-positive women with lymphedema, anti-WSP IgG levels were negatively correlated with the duration of lymphedema. The presence of anti-WSP IgG was also associated with the severity of inguinal adenopathy among men with hydrocele. In addition to the presence of anti-WSP antibodies among Haitians, 15 of 67 (22%) serum samples collected from individuals from North America, where filariasis is not endemic, were also positive for anti-WSP antibodies. In comparison to those from Haitians, anti-WSP antibodies from North Americans primarily recognized a distinct region of WSP located within the highly conserved second transmembrane domain. The results of this study demonstrate that anti-WSP antibody responses are associated with the presence of chronic filarial morbidity and not filarial infection status in humans and suggest that WSP should be further studied as a potential trigger for the development of filarial disease.