B cells maintain the homeostasis of splenic marginal zone antigen-presenting cells to promote the anti-viral CD8 ⁺ T cell response

Natural killer and CD8 ⁺ T cells are critical in the elimination of blood-borne viruses such as cytomegalovirus (CMV); however, the role of B cells in this process is less clear. Here, using the murine CMV (MCMV) infection model, we demonstrated that the B cell-deficient mice mounted a weaker primary virus-specific CD8 ⁺ T cell response than their wild-type counterparts, which was associated with increased viral transcription. Notably, we found that the contribution of B cells to the CD8 ⁺ T-cell-mediated anti-viral response was not associated with their ability to generate antibodies but with their ability to sustain Langerin ⁺ type 1 conventional dendritic cells (cDC1s), a dendritic cells (DC) subset known for being involved in viral and bacterial clearance in the marginal zone of the spleen. Furthermore, we found that the presence of Langerin ⁺ cDC1s is dependent on B cells expressing lymphotoxin (LTβ) to maintain CD169 ⁺ marginal metallophilic macrophages (MMMs). We further discovered, using ligand-receptor interaction analyses, that the communication between MMMs and Langerin ⁺ cDC1s was mediated via VCAM1 - ITGA4/ITGB1 interaction. Thus, our data reveals that B cell regulate the development of MMMs in the spleen via LTβ expression and consequently sustain Langerin ⁺ cDC1s homeostasis for effective initiation of an anti-viral CD8 ⁺ T cell response. Overall, our study offers a new perspective on how B cells maintain the homeostasis of antigen-presenting cells in the splenic marginal zone and thus indirectly affect the virus-specific CD8 ⁺ T cell response, which could potentially be extended to other infectious and autoimmune diseases as well as tumors.