Crucial role of Mer tyrosine kinase in the maintenance of SIGN-R1⁺ marginal zone macrophages

Mer Tyrosine Kinase receptor (Mer) is involved in anti-inflammatory efferocytosis. Here we report elevated spontaneous germinal center (Spt-GC) responses in Mer-deficient mice (Mer^−/−) that are associated with the loss of SIGN-R1⁺ marginal zone macrophages (MZMs). The dissipation of MZMs in Mer^−/− mice occurs independently of reduced cellularity or delocalization of marginal zone B cells, sinusoidal cells or of CD169⁺ metallophillic macrophages. We find that MZM dissipation in Mer^−/− mice contributes to apoptotic cell (AC) accumulation in Spt-GCs and dysregulation of the GC checkpoint, allowing an expansion of DNA-reactive B cells in GCs. We further observe that bone marrow derived macrophages from Mer^−/− mice produce more TNFα, and are susceptible to cell death upon exposure to ACs compared to WT macrophages. Anti-TNFα Ab treatment of Mer^−/− mice is, however, unable to reverse MZM loss, but results in reduced Spt-GC responses, indicating that TNFα promotes Spt-GC responses in Mer^−/− mice. Contrary to an anti-TNFα Ab treatment, treatment of Mer^−/− mice with a synthetic agonist for the transcription factor LXRα rescues a significant number of MZMs in vivo. Our data suggest that Mer-LXRα signaling plays an important role in the differentiation and maintenance of MZMs, which in turn regulate Spt-GC responses and tolerance.