Leishmania donovani Exploits Tunneling Nanotubes for Dissemination and Propagation of B Cell Activation

Polyclonal B cell activation and the resulting hypergammaglobulinemia are a detrimental consequence of visceral leishmaniasis (VL); however, the mechanisms underlying this excessive production of nonprotective antibodies are still poorly understood. Here, we show that a causative agent of VL, Leishmania donovani, induces CD21-dependent formation of tunneling nanotubule (TNT)-like protrusions in B cells. These intercellular connections are used by the parasite to disseminate among cells and propagate B cell activation, and close contact both among the cells and between B cells and parasites is required to achieve this activation. Direct contact between cells and parasites is also observed in vivo, as L. donovani can be detected in the splenic B cell area as early as 14 days postinfection. Interestingly, Leishmania parasites can also glide from macrophages to B cells via TNT-like protrusions. Taken together, our results suggest that, during in vivo infection, B cells may acquire L. donovani from macrophages via TNT-like protrusions, and these connections are subsequently exploited by the parasite to disseminate among B cells, thus propagating B cell activation and ultimately leading to polyclonal B cell activation. IMPORTANCE Leishmania donovani is a causative agent of visceral leishmaniasis, a potentially lethal disease characterized by strong B cell activation and the subsequent excessive production of nonprotective antibodies, which are known to worsen the disease. How Leishmania activates B cells is still unknown, particularly because this parasite mostly resides inside macrophages and would not have access to B cells during infection. In this study, we describe for the first time how the protozoan parasite Leishmania donovani induces and exploits the formation of protrusions that connect B lymphocytes with each other or with macrophages and glides on these structures from one cell to another. In this way, B cells can acquire Leishmania from macrophages and become activated upon contact with the parasites. This activation will then lead to antibody production. These findings provide an explanation for how the parasite may propagate B cell activation during infection.

LCMV-specific CD4 T cell dependent polyclonal B-cell activation upon persistent viral infection is short lived and extrafollicular

Persistent virus infections with non‐ or poorly cytopathic viruses are commonly associated with B cell dysregulations. These include the induction of hypergammaglobulinemia and the emergence of virus‐unspecific antibodies. These seemingly unspecific antibody responses interfere with the virus‐specific humoral immunity and contribute to delayed virus control. Whether these virus‐unspecific antibodies are induced in the B cell follicle or at extrafollicular sites and whether one specific CD4 T cell subset is involved in the polyclonal B cell activation is unclear. Here we studied virus‐unrelated IgG antibody responses against self or foreign antigens in the context of persistent lymphocytic choriomeningitis virus (LCMV) infection. We found that the LCMV‐unspecific antibody response is short‐lived and induced predominantly at extrafollicular sites and depends on the presence of LCMV‐specific CD4 T cells. Our data support a scenario in which activated, virus‐specific CD4 T cells provide help to non‐specific B cells at extrafollicular sites, supporting the production of virus unspecific IgG antibodies during persistent viral infection.

The Deadly Dance of B Cells with Trypanosomatids

B cells are notorious actors for the host's protection against several infectious diseases. So much so that early vaccinology seated its principles upon their long-term protective antibody secretion capabilities. Indeed, there are many examples of acute infectious diseases that are combated by functional humoral responses. However, some chronic infectious diseases actively induce immune deregulations that often lead to defective, if not deleterious, humoral immune responses. In this review we summarize how Leishmania and Trypanosoma spp. directly manipulate B cell responses to induce polyclonal B cell activation, hypergammaglobulinemia, low-specificity antibodies, limited B cell survival, and regulatory B cells, contributing therefore to immunopathology and the establishment of persistent infections.

Epstein-Barr Virus Lytic Reactivation Activates B Cells Polyclonally and Induces Activation-Induced Cytidine Deaminase Expression: A Mechanism Underlying Autoimmunity and Its Contribution to Graves' Disease

Graves' disease is an autoimmune disease that results in and is the most common cause of hyperthyroidism, and the reactivation of persisting Epstein–Barr virus (EBV) in B lymphocytes induces the differentiation of host B cells into plasma cells. We previously reported that some EBV-infected B cells had thyrotropin receptor antibodies (TRAbs) as surface immunoglobulins (Igs), and EBV reactivation induced these TRAb+EBV+ cells to produce TRAbs. EBV reactivation induces Ig production from host B cells. The purpose of the present study was to examine total Ig productions from B cell culture fluids and to detect activation-induced cytidine deaminase (AID), nuclear factor kappa B (NF-κB), and EBV latent membrane protein (LMP) 1 in culture B cells during EBV reactivation induction and then we discussed the mechanisms of EBV reactivation-induced Ig production in relation to autoimmunity. We showed that the EBV reactivation induces the production of every isotype of Ig and suggested that the Ig production was catalyzed by AID through LMP1 and NF-κB. The results that the amount of IgM was significantly larger compared with IgG suggested the polyclonal B cell activation due to LMP1. We proposed the pathway of EBV reactivation induced Ig production; B cells newly infected with EBV are activated by polyclonal B cell activation and produce Igs through plasma cell differentiation induced by EBV reactivation. LMP1-induced AID enabled B cells to undergo class-switch recombination to produce every isotype of Ig. According to this mechanism, EBV rescues autoreactive B cells to produce autoantibodies, which contribute to the development and exacerbation of autoimmune diseases.