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The ability to mount an effective anti-tumor immune response requires coordinate control of CD4 T cell and CD8 T cell function by antigen presenting cells (APCs). Unfortunately, tumors create an immunosuppressive microenvironment that helps protect tumor cells from immune recognition. In many cases this defect can be traced back to a failure of APCs (most importantly dendritic cells (DCs)) to recognize, process, and present tumor antigens to T cells. In this review, we will summarize work addressing the role of different DC subsets in anti-tumor immunity and the various mechanisms used by tumor cells to suppress the ability of APCs to stimulate potent anti-tumor T cell responses.

The expression and turnover of Ag-specific peptide–MHC class II (pMHC-II) on the surface of dendritic cells (DCs) is essential for their ability to efficiently activate CD4 T cells. Ubiquitination of pMHC-II by the E3 ubiquitin ligase March-I regulates surface expression and survival of pMHC-II in DCs. We now show that despite their high levels of surface pMHC-II, MHC class II (MHC-II) ubiquitination–deficient mouse DCs are functionally defective; they are poor stimulators of naive CD4 T cells and secrete IL-12 in response to LPS stimulation poorly. MHC-II ubiquitination–mutant DC defects are cell intrinsic, and single-cell RNA sequencing demonstrates that these DCs have an altered gene expression signature as compared with wild-type DCs. Curiously, these functional and gene transcription defects are reversed by activating the DCs with LPS. These results show that dysregulation of MHC-II turnover suppresses DC development and function.

Type 2 diabetes is typified by insulin-resistance in adipose tissue, skeletal muscle, and liver, leading to chronic hyperglycemia. Additionally, obesity and type 2 diabetes are characterized by chronic low-grade inflammation. Membrane-associated RING-CH-1 (MARCH1) is an E3 ubiquitin ligase best known for suppression of antigen presentation by dendritic and B cells. MARCH1 was recently found to negatively regulate the cell surface levels of the insulin receptor via ubiquitination. This, in turn, impaired insulin sensitivity in mouse models. Here, we report that MARCH1-deficient (knockout; KO) female mice exhibit excessive weight gain and excessive visceral adiposity when reared on standard chow diet, without increased inflammatory cell infiltration of adipose tissue. By contrast, male MARCH1 KO mice had similar weight gain and visceral adiposity to wildtype (WT) male mice. MARCH1 KO mice of both sexes were more glucose tolerant than WT mice. The levels of insulin receptor were generally higher in insulin-responsive tissues (especially the liver) from female MARCH1 KO mice compared to males, with the potential to account in part for the differences between male and female MARCH1 KO mice. We also explored a potential role for MARCH1 in human type 2 diabetes risk through genetic association testing in publicly-available datasets, and found evidence suggestive of association. Collectively, our data indicate an additional link between immune function and diabetes, specifically implicating MARCH1 as a regulator of lipid metabolism and glucose tolerance, whose function is modified by sex-specific factors.

March-I is a membrane-bound E3 ubiquitin ligase belonging to the membrane-associated RING-CH (March) family. March-I ubiquitinates and down-regulates the expression of major histocompatibility complex (MHC) class II and cluster of differentiation 86 (CD86) in antigen-presenting cells. March-I expression is regulated both transcriptionally and posttranslationally, and it has been reported that March-I is ubiquitinated and that this ubiquitination contributes to March-I turnover. However, the molecular mechanism regulating March-I ubiquitination and the importance of March-I's E3 ligase activity for March-I ubiquitination are not fully understood. Here we confirmed that, although March-I is ubiquitinated, it is not ubiquitinated on a lysine residue, as a lysine-less March-I variant was ubiquitinated similarly as wildtype March-I. We found that March-I E3 ligase activity is not required for its ubiquitination and does not regulate March-I protein expression, suggesting that March-I does not undergo autoubiquitination. Knocking down ubiquitin-conjugating enzyme E2 D1 (Ube2D1) impaired March-I ubiquitination, increased March-I expression, and enhanced March-I–dependent down-regulation of MHC-II proteins. Taken together, our results suggest that March-I undergoes lysine-independent ubiquitination by an as yet unidentified E3 ubiquitin ligase that, together with Ube2D1, regulates March-I expression.

MHC class II molecules on APCs bind to and presents peptides to antigen-specific CD4 T cells. DCs are the most potent subset of APC and can activate naïve T cells. Immature DCs possess relatively low expression of the costimulatory molecule CD86 and have a rapid rate of turnover of MHC-II. This is due in part to ubiquitin-mediated degradation of MHC-II and CD86 by the transmembrane (TM) ubiquitin ligase March-I in immature DCs. Curiously, March-I activity is suppressed by CD83, a TM protein that is upregulated by DC activation. Despite the well-documented role of March-I in specifically regulating MHC-II and CD86 expression, little is known about how these substrates interact with March-I and how CD83 is able to suppress March-I function. In this study we have examined the importance of TM domain interactions in March-I function by replacing TM domain sequences in March-I or CD83 with that of ICAM-1. A March-I TM1 mutant had reduced ability to affect MHC-II expression and had no effect on CD86 expression. Binding studies demonstrated that that March-I TM1 mutant lost all ability to bind to CD86. Whereas wild-type CD83 was able to block March-I downregulation of MHC-II and CD86, replacement of the CD83 TM domain with that of ICAM-1 completely abrogated CD83 suppression of March-I function. Binding studies confirmed that the CD83 TM mutant had lost all ability to bind to March-I. Our data therefore suggest that March-I binds to MHC-II and CD86 by TM domain interactions and that CD83 blocks March-I function by competitively inhibiting the binding of March-I to MHC-II and CD86.