*Purpose: Anti-HLA antibodies, both pre-formed and de novo are associated with worse graft survival. Glomerular endothelial cells are the targets for antibody mediated rejection. We hypothesized that there are differences in the mechanisms of endothelial cell activation by of anti-HLA class I and II antibodies. Also, this may explain differences in pathogenicity.

*Methods: Human primary renal glomerular endothelial cells (HRGEC) were HLA typed. Cells at passage 3-6, were stimulated with Gamma Interferon for 48hours. Cells were then exposed to sera with cell HLA specific anti-HLA class I and II antibodies in separate experiments for a serial time period starting from 2 minutes to maximum of 120minutes. Cell lysates were collected and studied for Phospho AKT and MAP Kinases by Western Blotting.

*Results: In these experiments, we demonstrate differential activation of phosphokinases in the presence of anti-HLA class I and class II antibodies.With anti-HLA class I antibodies progressive activation of phospho AKT and MAP Kinases were noted beginning at 15 minutes[Figure 1a]. Anti-HLA class II antibodies caused activation phospho AKT for 10minutes beginning at 2 minutes, then activation was not seen till 60 minutes. MAPK were activated from 2-10 minutes with no further activation [Figure 1b].

*Conclusions: This differential activation may lead to different downstream pathways that determine the endothelial cell susceptibility. Studies have shown that Phospho AKT activation promotes cell growth and prevents apoptosis. Continuous activation of pAKT by class I antibodies in contrast to class II antibodies may stimulate cell survival signals. Further studies on endothelial cell signaling pathways are needed to elucidate specific markers that are upregulated.

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