A subgroup of lupus patients with nephritis, innate T cell activation and low vitamin D is identified by the enhancement of circulating MHC class I‐related chain A

Summary The major histocompatibility complex (MHC) class I‐related chain A (MICA) is induced upon stress, and labels malfunctioning cells for their recognition by cytotoxic lymphocytes. Alterations in this recognition and also abnormal natural killer (NK) functions have been found in systemic lupus erythematosus (SLE). MICA can be shed from cells, subsequently acting as a soluble decoy receptor (sMICA). Our purpose was to study circulating sMICA levels in relationship with the activation of innate pathways in PBMC in a cohort of lupus patients. NK cells were characterized by flow cytometry. Gene expression of Toll‐like receptors (TLR), interferon (IFN)‐I sensitive genes and MICA were separately analyzed in monocytes, T cells and B cells. Serum sMICA was measured with enzyme‐linked immunosorbent assay (ELISA). In our cohort, NK cell counts dropped in relationship with disease activity. sMICA showed an inverse trend with NK cell counts, as well as a significant association with activity indices, but not with complement decrease. Levels of sMICA associated to proteinuria and active nephritis. A multivariate regression model revealed anti‐nuclear antibody (ANA) titres, the up‐regulation of TLR‐4 in T cells and lower vitamin D as predictors of sMICA enhancement. Interestingly, vitamin D showed an inverse association with proteinuria and a strong correlation with T cell MICA mRNA levels. According to our data, circulating sMICA identifies a subgroup of lupus patients with low vitamin D, innate activation of T cells and nephritis. We propose that lymphocyte shedding could account for the enhancement of sMICA and reflect an immune evasion mechanism driving disease activation in lupus. In a cross‐sectional study we identified a specific subgroup of lupus patients with enhanced levels of circulating soluble MICA (sMICA) – which is thought to act as a decoy receptor for NK cells – higher disease activity scores and active nephritis. The patients might have impaired NK functions, as suggested by the inverse correlation between NK cell counts and sMICA and the lack of association with complement activation. A robust relationship between sMICA and decreased vitamin D suggests that the vitamin holds up the balance of MICA homeostatic functions, which could be lost in these patients during flares.