Although B cells produce cytokines it is not known whether B cells can differentiate into effector subsets that secrete polarized arrays of cytokines. We have identified two populations of "effector" B cells (Be1 and Be2) that produce distinct patterns of cytokines depending on the cytokine environment in which the cells were stimulated during their primary encounter with antigen and T cells. These effector B cell subsets subsequently regulate the differentiation of naïve CD4+ T cells to TH1 and TH2 cells through production of polarizing cytokines such as interleukin 4 and interferon gamma. In addition, Be1 and Be2 cells could be identified in animals that were infected with pathogens that preferentially induce a Type 1 and Type 2 immune response. Together these results suggest that, in addition to their well defined role in antibody production, B cells may regulate immune responses to infectious pathogens through their production of cytokines.

T lymphocytes and gamma interferon (IFN-␥

Resistance to infection with Toxoplasma gondii was studied in mice lacking CD4 expression. Such mice developed more brain cysts and survived for a shorter time than did wild-type controls after peroral infection with ME49 cysts. After immunization with the ts-4 strain of T. gondii, CD4-deficient mice exhibited impaired resistance to a challenge infection with virulent RH tachyzoites. Thus, deficient CD4 expression increases the susceptibility of mice to a primary peroral T. gondii infection with cysts and impairs their ability to be successfully vaccinated. CD8 ؉ T cells from blood or spleens of Toxoplasma-infected, CD4-deficient mice expressed markers of activation at frequencies similar to those of infected wild-type mice. Production of IFN-␥ in vitro was moderately depressed, and levels of Toxoplasma-specific immunoglobulin G2a in serum were substantially lower than in wild-type mice. Administration of Toxoplasma-immune serum to ts-4-vaccinated CD4-deficient mice significantly improved their resistance to RH challenge. Also, the survival of CD4-deficient mice chronically infected with ME49 was significantly prolonged by administration of immune serum. These results demonstrate that in addition to CD8؉ T cells and IFN-␥, which are known to be critical for resistance, CD4؉ cells also contribute significantly to protection against chronic T. gondii infections and against challenge infections with highly virulent tachyzoites in immunized mice via their role as helper cells for production of isotype-switched antibodies.

The generation of effector, IFN-γ producing T lymphocytes and their accumulation at sites of infection are critical for host protection against various infectious diseases. The activation and differentiation of naive T lymphocytes into effector memory cells starts in lymphoid tissues, but it is not clear whether the Ag-experienced cells that leave lymph nodes (LN) are mature or if they undergo further changes in the periphery. We have previously shown that CD44highCD62Llow effector CD4 T lymphocytes generated during the course of mycobacterial infection can be segregated into two subsets on the basis of CD27 receptor expression. Only the CD27low subset exhibited a high capacity for IFN-γ secretion, indicating that low CD27 expression is characteristic of fully differentiated effector CD4 T lymphocytes. We demonstrate now that CD27low IFN-γ-producing CD4 T lymphocytes accumulate in the lungs but are rare in LNs. Several factors contribute to their preferential accumulation. First, CD27low CD4 T lymphocytes present in the LN are highly susceptible to apoptosis. Second, circulating CD27low CD4 T cells do not enter the LN but efficiently migrate to the lungs. Third, CD27high effector CD4 T cells that enter the lungs down-regulate CD27 expression in situ. In genetically heterogeneous mice that exhibit varying susceptibility to tuberculosis, the accumulation of mature CD27low CD4 T cells in the lungs correlates with the degree of protection against infection. Thus, we propose that terminal maturation of effector CD4 T lymphocytes in the periphery provides the host with efficient local defense and avoids potentially harmful actions of inflammatory cytokines in lymphoid organs.

SUMMARY Protection against tuberculosis depends upon the generation of CD4+ T cell effectors capable of producing IFN-g and stimulating macrophage antimycobacterial function. Effector CD4 + T cells are known to express CD44 hi CD62L lo surface phenotype.In this paper we demonstrate that a population of CD44 hi CD62L lo CD4 + effectors generated in response to Mycobacterium bovis BCG or M. tuberculosis infection in C57BL/6 mice is heterogeneous and consists of CD27 hi and CD27 lo T cell subsets. These subsets exhibit a similar degree of in vivo proliferation, but differ by the capacity for IFN-g production. Ex vivo isolated CD27 lo T cells express higher amounts of IFN-g RNA and contain higher frequencies of IFN-g producers compared to CD27 hi subset, as shown by real-time PCR, intracellular staining for IFNg and ELISPOT assays. In addition, CD27 lo CD4 + T cells uniformly express CD44 hi CD62L lo phenotype. We propose that CD27 lo CD44 hi CD62L lo CD4 + T cells represent highly differentiated effector cells with a high capacity for IFN-g secretion and antimycobacterial protection at the site of infection.