TLR agonists downregulate H2-O in CD8α− DCs1

Peptide loading of MHC class II (MHCII) molecules is catalyzed by the non-classical MHCII-related molecule, H2-M. H2-O, another MHCII-like molecule, associates with H2-M and modulates H2-M function. The MHCII presentation pathway is tightly regulated in dendritic cells (DCs); yet how the key modulators of MHCII presentation, H2-M and H2-O, are affected in different DC subsets in response to maturation is unknown. Here we show that H2-O is markedly downregulated in vivo in mouse CD8α − DCs in response to a broad array of TLR agonists. In contrast, CD8α + DCs only modestly downregulated H2-O in response to TLR-agonists. H2-M levels were slightly down-modulated in both CD8α − and CD8α + DCs. As a consequence, H2-M:H2-O ratios significantly increased for CD8α − but not CD8α + DCs. The TLR-mediated downregulation was DC-specific, as B cells did not show significant H2-O and H2-M downregulation. TLR4 signaling was required to mediate DC H2-O downregulation in response to LPS. Finally, our studies showed that the mechanism of H2-O downregulation was likely due to direct protein degradation of H2-O as well as down regulation of H2-O mRNA levels. The differential H2-O and H2-M modulation after DC maturation support the proposed roles of CD8α − dendritic cells in initiating CD4-restricted immune responses by optimal MHCII presentation and CD8α + DCs in promoting immune tolerance via presentation of low levels of MHCII-peptide.