CLL CELLS BIND AND PRESENT THE ERYTHROCYTE PROTEIN BAND 3

GALLETTI JEREMÍAS; MORANDE PABLO; CAÑONES CRISTIAN; BORGE MERCEDES; NANNINI PAULA; BEZARES FERNANDO; GAMBERALE ROMINA; GIORDANO MIRTA

Londres

Workshop; International Workshop on Chronic Lymphocytic Leukemia; 2007

The mechanisms underlying the frequent association between chroniclymphocytic leukemia (CLL) and autoimmune hemolytic anemia (AHA) remain illdefined. In AHA auto-antibodies are mostly directed to the erythrocyte proteinband 3 (B3), Rh complex or glycophorins. We explored the possibility that CLL cellscould initiate the autoimmune response by presenting B3 to T cells in the context ofappropriate co-stimulation. B3 was evaluated by flow cytometry in peripheralblood mononuclear cells (PBMC) from 19 CLL samples. We found that CLL cellsand monocytes bound biotinylated B3 to a very high extent compared to thenegligible levels in the T and NK cell populations. B3 binding to CLL cells was dosedependent(starting a 0,5 ?g/ml), showed evidence of saturability (plateauconcentrations ranged from 25-50 ?g/ml) and could be partially displaced byexcess untagged protein. Moreover, a monoclonal antibody directed to theamino-terminal region of the erythrocyte protein (mouse IgG2a, clone BIII-136)completely abrogated the binding phenomenon, indicating that this part of B3 isresponsible for the interaction with the leukemic cells. The binding site for B3 in CLLcells was sensitive to protease and neuraminidase activity suggesting theinvolvement of a membrane glycoprotein. Given that CLL cells were capable ofinternalizing bound B3, they were the evaluated as antigen presenting cells for B3.To this aim, monocyte-depleted PBMC were cultured in eh presence of B3 (10?g/ml) for 5-7 days, but no positive proliferative responses were observed in any of10 CLL samples analyzed. In an attempt to enhance their antigen-presentingcapacity, CLL cells were stimulated by CD40 engagement and pulsed with B3overnight instead of leaving the antigen throughout the culture. Under theseconditions, CLL cells became capable of stimulating specific T cell responses inabout 50% of the samples studied (stimulation index higher than 2, n=11). Ourresults show that CLL cells can specifically bind and capture B3 and, after bindactivated, they can induce T cell proliferation. We propose that the leukemicclone could initiate the auto-aggressive response to erythrocytes bay acting as apopulation of B3 presenting cells.