Inhibition of Src kinases combined with CD40 ligand blockade prolongs murine cardiac allograft survival

Members of the Src family of tyrosine kinases (SFKs) are requisite signaling molecules activated by multiple receptors during immune responses. Their expression and catalytic activity has not been characterized in allograft rejection in vivo. We measured expression and catalytic activity of SFKs in MHC- mismatched murine cardiac allografts. We also examined the effects of a Src inhibitor (CGP77675) with or without anti-CD154 mAb on graft survival, histology, and expression and catalytic activity of SFKs within the grafts. In acutely rejecting allografts from untreated controls, total activity of Hck and Lyn increased 10-fold, predominantly reflecting increases in the amount of protein. Total activity of Lck increased only fourfold, reflecting small changes in both the amount of protein and specific activity. One dose of anti-CD154 plus CGP77675 markedly diminished cellular infiltration, but survival was only moderately prolonged despite inhibition of all SFKs in the rejected grafts. Two doses of anti-CD154 plus CGP77675 allowed permanent graft acceptance in 60% of recipients even after discontinuation of the inhibitor. Both rejected and long surviving grafts showed increased activity of all SFKs. Recipients that rejected their grafts showed serum alloantibody production, and grafts rejected during treatment demonstrated deposition of complement indicating the contribution of antibody to rejection. The myeloid and B cell Src family kinases, Hck and Lyn, rather than the T cell Src kinase Lck, show the greatest increase in expression and total activity in rejecting allografts. Both rejected and long-surviving grafts show significant increases in SFK expression and acitivity.