B cells maintain the homeostasis of splenic marginal zone antigen-presenting cells to promote the antiviral 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 a murine CMV (MCMV) infection model, we demonstrated that B-cell-deficient mice mounted a weaker primary virus-specific CD8+ T-cell response than their wild-type counterparts did, which was associated with increased viral transcription. Notably, we found that the contribution of B cells to the CD8+ T-cell-mediated antiviral 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 cell (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, via ligand‒receptor interaction analyses, that the communication between MMMs and Langerin+ cDC1s was mediated via the VCAM1-ITGA4/ITGB1 interaction. Thus, our data reveal that B cells regulate the development of MMMs in the spleen via LTβ expression and consequently sustain Langerin+ cDC1 homeostasis for effective initiation of an antiviral 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 be extended to other infectious and autoimmune diseases as well as tumors.