Determining a role for CD45 in dendritic cells

CD45 is a leukocyte specific protein tyrosine phosphatase present on the surface of all nucleated, hematopoietic cells. Despite its well-characterized role in antigen receptor signaling, little is known about its function in cell types like dendritic cells (DCs). DCs are crucial to the immune response both for its initiation and for its suppression. In this dissertation, the effects of the lack of CD45 on dendritic cell development and function were studied. The most important finding was that the lack of CD45 had a differential impact on the proinflammatory cytokine profiles elicited in DCs by different TLR agonists. TLR4 ligation led to a decrease in proinflammatory cytokine and IFNβ production whereas stimulation through TLR2 or TLR9 increased cytokine production. This suggests CD45 may be acting as a negative regulator of MyD88-dependent cytokine signaling and a positive regulator of the Trif pathway. The absence of CD45 caused alterations in the phosphotyrosine levels of several Src family kinases including Lyn. In CD45-/- DCs, Lyn was not activated upon LPS stimulation and several substrates of Lyn that appear as negative regulators in the MyD88-dependent pathway of TLR4 signaling are also not phosphorylated, providing evidence that CD45 may be a negative regulator of this pathway. The absence of CD45 in TLR activated DCs had an effect on the IFNγ secretion by CD4+ T cells and NK cells, consistent with the cytokine profiles of the DCs These data demonstrate that modulation of TLR signaling by CD45, in DCs, has the ability to impact the development of the adaptive immune response. The absence of CD45 in mice did not result in increased survival upon challenge with a high dose of LPS. Serum TNFα levels were increased in the CD45-/- mice and they showed more severe symptoms of septic shock. However, the CD45-/- mice were also found to have an increase in the number of peritoneal macrophages. Overall this study shows that CD45 does play an important role in cell types other than lymphocytes. CD45 is a regulator of TLR-mediated cytokine secretion in DCs and thus directs the outcome of the adaptive immune response.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat