B Cells Protect T Cell-deficient mice from Cryptococcal Brain Invasion

In this issue of Virulence, Dufaud et al utilize a Rag1deficient murine model and adoptive transfers of B-cells or IgM-containing serum to directly examine the role of B-cells in host defense against the opportunistic yeast fungus Cryptococcus neoformans (CN). Cryptococcal meningoencephalitis remains a leading cause of mortality in patients with HIV/AIDS resulting in »200,000 annual deaths worldwide, hence, better understanding of the cellular and molecular factors that protect the host from cryptococcosis could lead to immune-based therapies and better patient outcomes. The susceptibility of patients with AIDS and idiopathic CD4+ T-cell lymphocytopenia to cryptococcosis clearly underscores the critical contribution of CD4+ T-cells in effective host defense. Moreover, macrophages are critical in promoting fungal clearance following their priming and effective cross-talk with T-cells via the production of IFN-gamma. On the other hand, the role of B-cells in anti-CN host defense has remained less understood. Prior studies in C57BL/6 mice depleted of B-1 cells, in sIgM¡/¡ mice that lack secreted IgM, and in X-linked immunodeficient mice (XID) that possess a mutation in the Bruton’s tyrosine kinase (Btk), have suggested a contribution of B-1 cells or na€ıve serum IgM in protection against cryptococcal brain invasion; however, definitive conclusions about the direct role of B-cells in anti-CN immunity were confounded in all of the aforementioned studies by the presence of T cells and because of associated defects in cellular immunity (XID mice) and in B-cell development (sIgM¡/¡ mice). In the present study, the authors utilize a low-virulence strain of CN, which upon infection of C57BL/6 mice results in a significant chronic pulmonary infection with only low-level dissemination and efficient local control of the fungus in the brain. In contrast to C57BL/6 mice, Dufaud el al show that CN-infected Rag1-deficient mice (on the C57BL/6 genetic background), which lack both T and B-cells, display a marked increase in fungal proliferation in the brain, which is particularly accentuated after the second week post-infection (Figure 1B). The authors restore (at least partly) B-cell function in Rag1-deficient hosts by adoptively transferring B-cells purified from the spleens of C57BL/6 mice into the Rag1-deficient animals. This transfer restored the numbers of B-1a and B-1b cells recruited to the CN-infected lungs, but not T-cells or B-2 cells, in agreement with previous reports of B-cell adoptive transfers in other inflammatory settings. Intriguingly, this B-cell transfer led to a marked reduction in the burden of CN in the brain of these mice compared to Rag1-deficient mice that did receive B-cells, a reduction in fungal load that reached levels comparable to those seen in CN-infected C57BL/6 mice. Instead, B-cell adoptive transfers in Rag1-deficient mice did not affect the fungal load in the CN-infected lung, highlighting a brain-specific requirement for B cell-dependent control of fungal load during cryptococcosis. This brain-specific role of B-cells in protection against cryptococcosis is is on par with recent clinical reports of brain infections by CN (and other fungi including Aspergillus) in patients receiving the small-molecule irreversible BTK inhibitor ibrutinib, which impairs B-cell receptor signaling, although patients with inherited BTK deficiency have not been reported to develop brain cryptococcal disease thus far. The results presented by Dufaud and colleagues are the most definitive data to date linking B-cells and cryptococcal host defense in the brain, especially in light of the demonstrated ability of B-cells to promote protection despite the absence of T-cells. Yet, important questions remain with regard to the mechanism(s) by which the transferred B-cells exert their protective effects and reduced CN invasion in the brain. While B-cells are best known for their secretion of antibody, they also