Reticulocalbin1-mediated regulation of calcium homeostasis in naïve T lymphoncytes.

Reticulocalbin1 (RCN1), a calcium-binding protein localized in the endoplasmic reticulum (ER), is implicated in cancer progression, but its role in the immune system remains poorly understood. To clarify the function of RCN1, we generated a RCN1-deficient (Rcn1-/-) mice using CRISPR-Cas9 system. Immunological characterization by flow cytometry revealed that while T cell populations in the spleen were unaffected, the proportion of CD8 single-positive (SP) thymocytes was significantly reduced in Rcn1-/- mice. In contrast, in vitro stimulation of splenic CD8+ T cells revealed no significant differences in the expression of cell surface activation markers or cytokines between Rcn1+/+ and Rcn1-/- mice. Functional analysis showed that at baseline, cytosolic calcium levels were significantly higher in Rcn1+/+ CD8+ T cells. In contrast, stimulus-induced calcium responses, including ER calcium release, extracellular calcium entry, and TCR-dependent calcium flux, were preserved in both genotypes. Collectively, our results suggest that RCN1 contributes to the fine-tuning of ER-associated calcium handling in naïve (non-activated) T cells, thereby modulating basal cytosolic calcium levels and supporting efficient thymic CD8⁺ T cell development.