CLL cells bind and present the erythrocyte protein band 3.

JEREMIAS GALLETI; MORANDE PABLO; CAÑONES CRISTIAN; BORGE MERCEDES; NANNINI PAULA ROMINA; BEZARES FERNANDO; GAMBERALE ROMINA; MIRTA GIORDANO

Londres, Inglaterra.

Workshop; XII International Workshop on CLL; 2007

The mechanisms underlying the frequent association between chronic lymphocytic leukemia (CLL) and autoimmune hemolytic anemia (AHA) remain ill defined. In AHA autoantibodies are mostly directed to the erythrocyte proteins band 3 (B3), Rh complex or glycophorins. We explored the possibility that CLL cells could initiate the autoimmune response by presenting B3 to T cells in the context of appropriate costimulation. B3 binding was evaluated by flow cytometry in peripheral blood mononuclear cells (PBMC) from 19 CLL samples. We found that CLL cells and monocytes bound biotinylated B3 to a very high extent compared to the negligible levels in the T and NK cell populations. B3 binding to CLL cells was dose-dependent  (starting at 0,5 ug/ml), showed evidence of saturability and could be partially displaced by excess untagged protein. Moreover, a monoclonal antibody directed to the amino-terminal region of the erythrocyte protein completely abrogated the binding phenomenon, indicating that this part of B3 is responsible for the interaction with the leukemic cells. The binding site for B3 in CLL cells was sensitive to protease and neuraminidase activity suggesting the involvement of a membrane glycoprotein.  Given that CLL cells were also capable of internalizing bound B3, they were then evaluated as antigen presenting cells for B3. To this aim, monocyte-depleted PBMC were cultured in the presence of B3 (10 ug/ml) for 5-7 days, but no positive proliferative responses were observed in any of 11 CLL samples analyzed. In an attempt to enhance their antigen-presenting capacity, CLL cells were stimulated by CD40 engagement and pulsed with B3 instead of leaving the antigen throughout the culture. Under these conditions, CLL cells became capable of stimulating specific T cell responses in about 50% of the samples studied. Our results show that CLL cells can specifically bind and capture B3 and, after being activated, they can induce T cell proliferation. We propose that the leukemic clone could initiate the autoaggressive response to erythrocytes by acting as a population of B3 presenting cells.