Deficiency In Stress Response Gadd45a and Gadd45b Alters MAPK Kinase Signaling Leading to Functional Defects In Innate Immune Responses of Neutrophils and Macrophages In Vitro and In Vivo

Abstract 927

Stress sensor GADD45 proteins modulate p38-NF-Kb and JNK signaling; which play major roles in leukocyte activation and innate immunity. We have previously documented that GADD45a-deficient and GADD45b-deficient mice exhibit impaired responses to hematological stress, including acute stimulation with inflammatory cytokines (Gupta et. al, Oncogene. 25:5537, 2006). Whether GADD45 genes modulate myeloid compartment function, notably the response of macrophages & granulocytes to inflammatory stress, remains largely unexplored. Data obtained thus far indicate that bone marrow (BM) derived neutrophils and macrophages lacking GADD45 family members exhibit defects in chemotaxis towards inflammatory stimuli. BM derived neutrophils and macrophages lacking GADD45 family members were also observed to exhibit defects in the production of reactive oxygen species (ROS) upon treatment with LPS (500ng/mL) in vitro. Furthermore, BM derived macrophages from mice lacking GADD45a and GADD45b were found to be defective in phagocytosis and cellular adhesion upon stimulation with LPS (500ng/mL) in vitro. Additionally, it was observed that BM derived neutrophils and macrophages lacking GADD45 family members exhibit defects in inflammatory cytokine secretion in response to activation with LPS. Notably, GADD45a, GADD45b & GADD45g null mice injected intraperitoneally with sublethal doses of LPS were significantly more susceptible to septic shock compared to wt mice, showing significantly increased morbidity. Moreover at 18 hrs. post-injection, the spleens of KO mice harbored apoptotic foci in the white pulp, identified as tingible body macrophages ingesting dying cells. To dissect the signaling pathways involved, MAPK specific chemical inhibitors were used to correlate MAPK signaling alteration to defects in innate immune responses of neutrophils and macrophages lacking GADD45a or GADD45b. LPS stimulated wt neutrophils treated with MAPK inhibitors exhibited defects in signaling and innate immune responses mimicking neutrophils and macrophages lacking GADD45a or GADD45b. Collectively, these novel data provide evidence to show that deficiency in GADD45a and GADD45b leads to functional defects in the myeloid innate immune responses of neutrophils and macrophages in vitro and in vivo. These findings increase our understanding of innate immunity and have the potential to contribute towards development of novel approaches to clinical treatment of sepsis.