Endosomal LRBA regulates the endo-lysosomal pathway

Deleterious mutations in the LRBA (Lipopolysaccharide Responsive Beige-like Anchor protein) gene cause severe childhood immune dysregulation. The clinical manifestations of LRBA deficiency syndrome are highly variable. Thus, the complexity of the symptoms involving multiple organs and the broad range of unpredictable clinical manifestations complicate the choice of therapeutic interventions. Although LRBA has been linked to Rab11-dependent trafficking of the immune checkpoint protein CTLA-4, its precise cellular role remains elusive. We show that LRBA, however, does not colocalize with Rab11. Instead, LRBA is recruited by members of the small GTPase Arf protein family to the TGN and to Rab4+ endosomes, where it controls intracellular traffic. In patient-derived fibroblasts, loss of LRBA led to defects in the endosomal pathway yielding the accumulation of enlarged endolysosomes. Thus, LRBA appears to regulate flow through the endosomal system on Rab4+ endosomes. Our data strongly suggest functions of LRBA beyond CTLA-4 trafficking and provide a conceptual framework to develop new therapies for LRBA deficiency. Summary LRBA-deficient patients exhibit enlarged functional endolysosomes due to defects in recycling to the plasma membrane. LRBA localization on Rab4+ endosomes depends on Arf1 and Arf3, and is essential for a functional endosomal-lysosomal pathway. Our results could inform new treatment options. ### Competing Interest Statement The authors have declared no competing interest.