Emily Kugelmann

Rac2 Hyperactivity Drives Neutrophil Degranulation, Myeloperoxidase Deficiency, and T-Cell Lymphopenia

Faculty PI: Denise J. Montell

Patients with a dominant hyperactivating mutation in Rac2, Rac2E62K, suffer from recurrent infections and severe T-cell lymphopenia—the causes of which are currently unknown. These patients have normal neutrophil counts yet remain susceptible to S. aureus, Aspergillus, and sinopulmonary infections, indicating a neutrophil killing defect. Rac2 drives cytoskeletal remodeling in hematopoietic cells. Rac2 regulates how neutrophils (white blood cells that defend against infection) engulf targets and release primary granules (lysosomal packages that carry antimicrobial materials to destroy engulfed pathogens). Utilizing a Rac2+/E62K mouse model that recapitulates patient phenotypes, we hypothesize that the Rac2+/E62K mutation may compromise antimicrobial function in neutrophils. We have found that Rac2+/E62K murine neutrophils have normal development and egress from the bone marrow as well as enhanced engulfment of dead bacteria. However, intracellular staining and microscopy show decreased levels of Myeloperoxidase (MPO), a non-replenishable enzyme in primary granules necessary to kill engulfed pathogens. Moreover, through flow cytometry, we see that Rac2+/E62K neutrophils show a 2-fold increase in cell-surface CD63, a well-established marker that becomes exposed after primary degranulation. Consistent with this, when stimulated with fMLP (a bacterial peptide that triggers an immune response) and measured for CD63, Rac2+/E62K neutrophils degranulate at 1 µM fMLP while wildtype neutrophils show minimal response even at 100 µM. Based on these findings, we postulate that Rac2+/E62K primes neutrophils for premature primary degranulation, expending MPO and rendering neutrophils unable to kill engulfed pathogens. Given this, we aim to continue exploring Rac2E62K patient phenotypes and their potential immune therapy applications.