LRBA drives cytoskeleton dynamics in B and T cells
SINDRAM E. 1,2,3, ZEIDLER V. 1,4, MALFERTHEIMER K. 1,4, RUSH-KITTLE J. 1,3, MACE E. 5, SPINDLER V. 6, GRIMBACHER B. 1,7,8,9,10, GÁMEZ-DÍAZ L. 1
1 Institute for Immunodeficiency, Center for Chronic Immunodeficiency, Medical Center Freiburg, Freiburg, Germany; 2 Spemann Graduate School of Biology and Medicine (SGBM), University of Freiburg, Freiburg, Germany; 3 Faculty of Biology, University of Freiburg, Freiburg, Germany; 4 Faculty of Medicine, University of Freiburg, Freiburg, Germany; 5 Vagelos College of Physicians and Surgeons, Columbia University, New York, United States; 6 Department of Biomedicine, University of Basel, Basel, Switzerland; 7 Clinic of Rheumatology and Clinical Immunology, Center for Chronic Immunodeficiency (CCI), Medical Center, Faculty of Medicine, University of Freiburg, Freiburg, Germany; 8 DZIF – German Center for Infection Research, Satellite Center Freiburg, Freiburg, Germany; 9 CIBSS – Centre for Integrative Biological Signalling Studies, University of Freiburg, Freiburg, Germany; 10 RESIST – Cluster of Excellence 2155, Hannover Medical School, Satellite Center Freiburg, Freiburg, Germany
LPS-responsive beach-like anchor protein (LRBA) belongs to the BEACH-domain-containing protein (BDCP) family. BDCPs operate as scaffold proteins forming multiprotein complexes involved in vesicle trafficking and receptor signaling pathways. LRBA deficiency is an inborn error of immunity (IEI) caused by biallelic mutations in LRBA that abolish LRBA protein expression. This syndrome is characterized by recurrent infections associated with poor antibody response and multiple autoimmune manifestations. The latter can be explained by the role of LRBA in the trafficking of cytotoxic T lymphocyte antigen 4 (CTLA-4) containing vesicles from intracellular storages to the plasma membrane of regulatory T cells. However, the aberrant CTLA-4 trafficking does not explain the increased disease severity and mortality, earlier onset of symptoms and reduced response to Abatacept in LRBA-deficient patients.
Aiming to investigate additional roles of LRBA in immune cells we performed SILAC-IP-MS analysis in lymphoblastoid B-cells and in Jurkat T-cells, which revealed 81 potential protein interactors of LRBA, including eight proteins associated with cytoskeleton dynamics. Using proximity ligation assay (PLA) and co-immunoprecipitation (co-IP), LRBA was proven to interact with Myosin-9, which encodes the heavy chain of the non-muscle Myosin IIA (NMIIA). NMIIA is involved in cytoskeleton rearrangement during cell migration, receptor internalization and signaling, immune synapse formation, as well as Ca2+ release. In fact, we observed delayed BCR and TCR internalization and reduced bead phagocytosis in B- and T-cells lacking LRBA. Moreover, loss of LRBA in B and T cells resulted in an atypical immune synapse formation and abnormal chemotaxis towards CXCL12. The latter was found reduced in B-cells, whereas T-cells showed increased chemotaxis compared to their WT counterpart. This finding is in line with the poor B-cell response and the aberrant T-cell activation observed in LRBA-deficient patients. Taken together, our results indicate that LRBA is essential for cytoskeleton-dependent immune function in a cell-specific manner.