Critical Role of Hem1 in Hematopoietic Cell Chemotaxis and Innate Immunity to S. Pneumoniae

Hem1 is a hematopoietic specific member of the HEM family proteins, which have been identified as components of the WAVE regulatory complex. To further characterize the role of Hem1 in hematopoietic cell function, we have generated mice deficient in Hem1 using gene targeting methodology. Hem1-deficient mice manifest several phenotypic abnormalities, including peripheral blood lymphopenia and neutrophilia, splenomegaly, premature mortality, and with variable penetrance thymic hypoplasia and alopecia. Flow cytometric analysis of peripheral blood and spleen demonstrated that Hem1−/ − mice have a marked reduction in peripheral B cells, changes that were evident in both adult and neonatal mice; the splenomegaly in Hem1−/ − mice was attributable to increased extramedullary hematopoiesis. The frequency of bone marrow B cell progenitors was also markedly reduced in Hem1−/ − mice. To assess the role of Hem1 in hematopoietic stem cell (HSC) function, competitive bone marrow transplantation assays were performed. In contrast to wild-type HSCs, Hem1-deficient HSCs had poor competitive repopulating activity in irradiated recipient mice. KSL cell analysis demonstrated no significant difference in the frequency of lin-c-kit+Sca1+ HSCs between wild-type and Hem1−/ − bone marrow, suggesting that the defective competitive repopulating activity of Hem1−/ − HSCs is attributable to defective bone marrow homing or stem cell niche interaction. Given the biochemical evidence implicating the HEM proteins as a component of the WAVE regulatory complex, we also assessed the ability of Hem1-deficient leukocytes to undergo cytoskeletal remodeling in vitro. Using a transwell assay, Hem1−/ − bone marrow storage pool neutrophils demonstrated markedly blunted chemotactic responses to formylated peptide which was attributable to defective f-actin formation. Hem1−/ − peripheral CD4+ T cells similarly manifested chemotactic defects in response to SDF-1, and showed blunted proliferation when stimulated with antibodies against CD3 and CD28. Finally, a model of Streptococcus pneumoniae infection was employed to test the role of Hem1 in the in vivo function of neutrophils. Hem1−/ − mice were dramatically more sensitive to S. pneumoniae than wild-type littermates, as manifested by the inability to eliminate S. pneumoniae organisms in vivo and higher mortality. In summary, Hem1 deficiency results in deficiencies and functional defects in multiple hematopoietic lineages due to defective signaling to the actin cytoskeleton, and importantly, Hem1 plays a critical role in innate immunity to S. pneumoniae in vivo.