Ltbr -/- mice display altered B and T cell subpopulations in the bone marrow and peritoneal cavity after Toxoplasma gondii infection

Ltbr-deficient (Ltbr-/-) mice exhibit severe defects in innate and adaptive immunity against various pathogens, including Toxoplasma gondii. Intriguingly, Ltbr-/- mice are deficient in anti-Toxoplasma immunoglobulin (Ig) production and Ig class switch. In this study, we investigated immune cell populations in the bone marrow (BM) and peritoneal cavity (PerC) of Ltbr / mice before and during T. gondii infection, focusing on the B cell compartment. Strikingly, the Ltbr-/- BM was characterized by elevated B cell frequencies after infection, primarily due to mature B cells. Early differentiation stages of B cells were severely reduced in the BM of both genotypes after infection due to inflammation-induced reduction of Cxcl12 expression, whereas Ltbr-/- transitional and especially mature B cells exhibited resistance to reduction. Ltbr-/- BM plasma cells (PCs) showed similar or increased frequencies compared to WT counterparts, however with a predominant IgM+ rather than IgA+ surface isotype expression, and comparable frequencies of IgM- IgA- PCs. While the Ltbr-/- PerC also displayed increased B-2 and B-1b cell frequencies, intriguingly, B-1a frequencies remained comparable to WT counterparts, resulting in a distorted B-1a to B-1b cell ratio in Ltbr-/- mice. In addition to B cells, Ltbr-/- BM T cell subpopulations were also altered, showing increased CD4+ and reduced double negative (DN) T cell frequencies, whereas CD8+ T cell frequencies remained unaffected. T. gondii infection resulted in a delayed inflammatory response in Ltbr-/- compared to WT mice, as demonstrated after BM RNA sequencing. Additionally, MHCII+ proinflammatory (Ly6C+) monocytes were diminished in Ltbr-/- BM, whereas neutrophils, rather than T cells, dominated the Ltbr-/- peritoneal immune cell composition at day 9 post-infection. This study identified novel and distinct alterations in B and T cell differentiation, kinetics and activation phenotypes in Ltbr-deficient mice, identifying potential new pathways for further research to elucidate the specific roles of Ltbr signaling in pathogen elimination.