T cell-derived protein S engages TAM receptor signaling in dendritic cells to control the magnitude of the immune response.

Dendritic cell (DC) activation is essential for the induction of immune defense against pathogens, yet needs to be tightly controlled to avoid chronic inflammation and exaggerated immune responses. Here, we identify a mechanism of immune homeostasis by which adaptive immunity, once triggered, tempers DC activation and prevents overreactive immune responses. T cells, once activated, produced Protein S (Pros1) that signaled through TAM receptor tyrosine kinases in DCs to limit the magnitude of DC activation. Genetic ablation of Pros1 in mouse T cells led to increased expression of costimulatory molecules and cytokines in DCs and enhanced immune responses to T cell-dependent antigens, as well as increased colitis. Additionally, PROS1 was expressed in activated human T cells, and its ability to regulate DC activation was conserved. Our results identify a heretofore unrecognized, homeostatic negative feedback mechanism at the interface of adaptive and innate immunity that maintains the physiological magnitude of the immune response.

Ubiquitin-mediated regulation of CD86 protein expression by the ubiquitin ligase membrane-associated RING-CH-1 (MARCH1).

The activation of naïve T cells requires antigen presentation by dendritic cells (DCs), and the process of antigen presentation is regulated over the course of DC maturation. One key aspect of this regulation is the cell surface up-regulation upon DC maturation of peptide·MHC-II complexes and the costimulatory molecule CD86. It is now clear that these critical induction events involve changes in ubiquitin-dependent trafficking of MHC-II and CD86 by the E3 ligase membrane-associated RING-CH-1 (MARCH1). Although ubiquitin-dependent trafficking of MHC-II has been well characterized, much less is known regarding the post-transcriptional regulation of CD86 expression. Here, we examined the physical and functional interaction between CD86 and MARCH1. We observed that CD86 is rapidly endocytosed in the presence of MARCH1 followed by lysosome-dependent degradation. Furthermore, we found that the association between CD86 and MARCH1 was conferred primarily by the transmembrane domains of the respective proteins. In contrast to MHC-II, which has a single, conserved ubiquitin acceptor site in the cytosolic domain, we found that multiple lysine residues in the cytosolic tail of CD86 could support ubiquitination consistent with the relative lack of sequence conservation across species within the CD86 cytosolic domain. These findings suggest that MARCH1 recruits multiple substrates via transmembrane domain-mediated interactions to permit substrate ubiquitination in the face of diverse cytosolic domain sequences.

Discrete domains of MARCH1 mediate its localization, functional interactions, and posttranscriptional control of expression.

Within APCs, ubiquitination regulates the trafficking of immune modulators such as MHC class II and CD86 (B7.2) molecules. MARCH1 (membrane-associated RING-CH), a newly identified ubiquitin E3 ligase expressed in APCs, ubiquitinates MHC class II, thereby reducing its surface expression. Following LPS-induced maturation of dendritic cells, MARCH1 mRNA is down-regulated and MHC class II is redistributed to the cell surface from endosomal compartments. Here, we show that MARCH1 expression is also regulated at the posttranscriptional level. In primary dendritic cell and APC cell lines of murine origin, MARCH1 had a half-life of <30 min. MARCH1 degradation appears to occur partly in lysosomes, since inhibiting lysosomal activity stabilized MARCH1. Similar stabilization was observed when MARCH1-expressing cells were treated with cysteine protease inhibitors. Mutational analyses of MARCH1 defined discrete domains required for destabilization, proper localization, and functional interaction with substrates. Taken together, these data suggest that MARCH1 expression is regulated at a posttranscriptional level by trafficking within the endolysosomal pathway where MARCH1 is proteolyzed. The short half-life of MARCH1 permits very rapid changes in the levels of the protein in response to changes in the mRNA, resulting in efficient induction of Ag presentation once APCs receive maturational signals.