T. brucei Infection Reduces B Lymphopoiesis in Bone Marrow and Truncates Compensatory Splenic Lymphopoiesis through Transitional B-Cell Apoptosis

African trypanosomes of the Trypanosoma brucei species are extracellular protozoan parasites that cause the deadly disease African trypanosomiasis in humans and contribute to the animal counterpart, Nagana. Trypanosome clearance from the bloodstream is mediated by antibodies specific for their Variant Surface Glycoprotein (VSG) coat antigens. However, T. brucei infection induces polyclonal B cell activation, B cell clonal exhaustion, sustained depletion of mature splenic Marginal Zone B (MZB) and Follicular B (FoB) cells, and destruction of the B-cell memory compartment. To determine how trypanosome infection compromises the humoral immune defense system we used a C57BL/6 T. brucei AnTat 1.1 mouse model and multicolor flow cytometry to document B cell development and maturation during infection. Our results show a more than 95% reduction in B cell precursor numbers from the CLP, pre-pro-B, pro-B, pre-B and immature B cell stages in the bone marrow. In the spleen, T. brucei induces extramedullary B lymphopoiesis as evidenced by significant increases in HSC-LMPP, CLP, pre-pro-B, pro-B and pre-B cell populations. However, final B cell maturation is abrogated by infection-induced apoptosis of transitional B cells of both the T1 and T2 populations which is not uniquely dependent on TNF-, Fas-, or prostaglandin-dependent death pathways. Results obtained from ex vivo co-cultures of living bloodstream form trypanosomes and splenocytes demonstrate that trypanosome surface coat-dependent contact with T1/2 B cells triggers their deletion. We conclude that infection-induced and possibly parasite-contact dependent deletion of transitional B cells prevents replenishment of mature B cell compartments during infection thus contributing to a loss of the host's capacity to sustain antibody responses against recurring parasitemic waves

B lymphocytes promote the establishment of the plateau phase of brucellosis

Brucella abortus is a facultative intracellular gram-negative bacterium that causes chronic protracted infections of humans and livestock. It is also a potential biowarfare agent and thus development of vaccines for humans is an important goal. In mice infected with Brucella abortus, protective roles for CD4 and CD8 T cells have been demonstrated. A critical role for T cells is likely to be production of IFNγ as IFNγ-deficient mice succumb to brucellosis. Here we show both T cell subsets contribute to control of infection by virulent strain B. abortus 2308 by producing IFNγ and CD4 T cells but not CD8 T cells facilitate clearance of infection. Strikingly, antibody-independent B cell functions contribute to the development of the high plateau phase of infection as both B cell/antibody-deficient C57BL/6 and BALB/c mice cleared 99% of the infection by 2 weeks in an antibody-independent manner. Further characterization of the immune response revealed that B cells produced regulatory cytokines in both C57BL/6 and BALB/c mice. B cells from C57BL/6 mice produced IL-10 while B cells from BALB/c mice produced TGF-β, which in turn can dampen protective IFN-γ mediated immune responses. Consequently, clearance of infection in B cell deficient C57BL/6 mice coincided with a decrease in IL-10 and an increase in the proportion of IFNγ-producing T cells while the clearance of infection in B cell deficient BALB/c mice was associated with a decrease in TGF-β producing cells and CD4 T regulatory cells. Due to the rapid clearance of infection in B cell deficient mice, we tested whether B lymphocytes act as an infection niche for brucellae and found that IgM and complement-opsonized viable B. abortus were phagocytosed by and survived within B lymphocytes in vitro. Furthermore, live brucellae were harbored within B lymphocytes in infected mice and these cells transferred disease to uninfected animals. Throughout the infection, a greater proportion of infected B lymphocytes produced TGFβ compared to non-infected B cells. Thus, B lymphocytes contribute to chronic brucellosis, and possibly other diseases, by providing an intracellular niche that can shield the bacteria from the host immune system and dampen inflammatory responses needed to clear the infection

B cell-deficient mice display markedly enhanced resistance to the intracellular bacterium Brucella abortus

Background. Brucella species are facultative intracellular bacteria that cause lifelong infections in humans and livestock. Methods. Here we evaluated the contribution of B cells in control of murine brucellosis in the more susceptible BALB/c and the more resistant C57BL/6 mice by infecting B cell-deficient mice. Results. Strikingly, in the absence of B cells in both C57BL/6 and BALB/c mice, 99% and 99.5% of the infection found in wild type mice was cleared, respectively. This augmented clearance was not reversed in either strain by passive transfer of immune serum. In C57BL/6 mice, the clearance of infection coincided with an increase in interferon γ (IFN-γ)- producing CD4 and CD8 T cells and a reduction in interleukin 10 (IL-10)-producing cells. In BALB/c mice, this clearance was IFN-γ- dependent, as B cell/IFN-γ dual knockout mice were unable to clear the infection, and was inversely related to the levels of transforming growth factor β (TGF-β). Furthermore, B cells were found to produce TGF-β and IL-10 during early stages of infection in BALB/c wild-type and C57BL/6 wild-type mice, respectively. Conclusions. Thus, we demonstrate that the establishment of the high plateau phase of infection is dependent on non-antibody-mediated B cell effector mechanisms, including B regulatory functions, during murine brucellosis. © The Author 2011. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved

T Regulatory Cells Support Plasma Cell Populations in the Bone Marrow

Long-lived plasma cells (PCs) in the bone marrow (BM) are a critical source of antibodies after infection or vaccination, but questions remain about the factors that control PCs. We found that systemic infection alters the BM, greatly reducing PCs and regulatory T (Treg) cells, a population that contributes to immune privilege in the BM. The use of intravital imaging revealed that BM Treg cells display a distinct behavior characterized by sustained co-localization with PCs and CD11c-YFP+ cells. Gene expression profiling indicated that BM Treg cells express high levels of Treg effector molecules, and CTLA-4 deletion in these cells resulted in elevated PCs. Furthermore, preservation of Treg cells during systemic infection prevents PC loss, while Treg cell depletion in uninfected mice reduced PC populations. These studies suggest a role for Treg cells in PC biology and provide a potential target for the modulation of PCs during vaccine-induced humoral responses or autoimmunity

A role for IL-27p28 as an antagonist of gp130-mediated signaling

The heterodimeric cytokine interleukin 27 (IL-27) signals through the IL-27Rα subunit of its receptor, combined with gp130, a common receptor chain used by several cytokines, including IL-6. Notably, the IL-27 subunits p28 (IL-27p28) and EBI3 are not always expressed together, which suggests that they may have unique functions. Here we show that IL-27p28, independently of EBI3, antagonized cytokine signaling through gp130 and IL-6-mediated production of IL-17 and IL-10. Similarly, the ability to generate antibody responses was dependent on the activity of gp130-signaling cytokines. Mice transgenic for expression of IL-27p28 showed a substantial defect in the formation of germinal centers and antibody production. Thus, IL-27p28, as a natural antagonist of gp130-mediated signaling, may be useful as a therapeutic for managing inflammation mediated by cytokines that signal through gp130