BLOC1S1 regulates autolysosomal and exosomal dynamics during CD4⁺ T cell differentiation

Although the endolysosome system is central to intracellular recycling, signal transduction, and intercellular communication via exocytosis, its role in immunoregulation remains incompletely defined. We recently identified that CD4+ T cell-specific depletion of BLOC1S1, a component of multiprotein complexes regulating endolysosomal biology, predisposes toward type 2 (Th2) immunity. We therefore hypothesized that the study of BLOC1S1-deficient CD4+ T cells would expand our understanding of endolysosomal dynamics in Th2 function. Here, we demonstrate that CD4+ T cell BLOC1S1 deficiency resulted in aberrant lysosomal distribution, accumulation of endosomal vesicles, and increased exocytosis, which collectively correlated with enhanced Th2 immune responses. The phenotype was associated with upregulation of key components of the exocytosis machinery, including RAB11 and VAMP7. Functional inhibition of these vesicle trafficking proteins following siRNA knockdown of RAB11 and VAMP7 significantly attenuated Th2 cytokine secretion in BLOC1S1-deficient CD4+ T cells, highlighting their essential role in exosome-mediated cytokine export. Furthermore, exosomes derived from BLOC1S1-deficient CD4+ T cells promoted Th2 polarization in recipient cells, indicating a mechanism of intracellular amplification. Together, these findings identify BLOC1S1 as a critical regulator of lysosomal dynamics and exocytic vesicle fusion, thereby linking intracellular trafficking mechanisms to Th2 immune regulation.