Rituximab-mediated antibody-dependent cellular cytotoxicity against neoplastic B cells is stimulated strongly by interleukin-2.

BACKGROUND AND OBJECTIVES: We analyzed the sensitivity of freshly isolated neoplastic B cells to rituximab-mediated antibody-dependent cellular cytotoxicity (ADCC), using different effector cells. DESIGN AND METHODS: ADCC was performed by 51Cr release assays in vitro, using peripheral blood mononuclear cells, IL-2-activated or expanded NK cells, neutrophils or macrophages as effector cells. B lymphoma lines and freshly isolated leukemic samples were used as targets. RESULTS: NK cells, but PMN or macrophages mediated rituximab dependent cellular cytotoxicity against two B lymphoma lines. Purified NK cells (95% CD56+/CD16+) reached 70% lysis at the highest E:T ratio. By contrast, all freshly isolated B leukemia or lymphoma cases, including 5 chronic lymphocytic leukemia, 1 B-prolymphocytic leukemia, 1 mantle cell lymphoma, 2 marginal zone lymhomas and 2 follicular lymphomas were poorly lysed by ADCC in the same conditions and regardless of CD20 expression levels, reaching a mean of 4% and 27% maximal lysis with PBMC or purified NK cells, respectively. Interestingly, short term IL-2 cultured PBMC, containing 10 % activated NK cells, as well as long-term expanded NK cells, containing 80-95% activated NK cells, became strong ADCC effector cells with rituximab and lysed all leukemic samples to a mean of 57% and 67% at the highest E:T ratio, respectively. INTERPRETATION AND CONCLUSIONS: Primary leukemic cells are more resistant than cell lines to rituximab- and NK cell-mediated ADCC but short-term exposure to IL-2 or long-term expansion of NK cells in vitro may provide effective tools to improve the therapeutic activity of rituximab.

Acquired immunodeficiency syndrome-associated lymphomas are efficiently lysed through complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity by rituximab.

Rituximab (Mabthera) and alemtuzumab (Campath(R), Mabcampath(R)) are non-conjugated IgG1 therapeutic monoclonal antibodies directed against the CD20 and CD52 surface antigens respectively. They are presently used in the therapy of indolent B-cell non-Hodgkin's lymphoma (B-NHL) and of B-cell chronic lymphocytic leukaemia, and are thought to act mainly through complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). Here we have analysed the capacity of these two monoclonal antibodies to lyse cell lines of acquired immunodeficiency syndrome (AIDS)-related B-NHL through either complement activation or antibody-dependent cytotoxicity. Rituximab strongly activated both CDC and ADCC against CD20-positive AIDS-NHL cells lines, inducing up to 60-98% and 20% specific lysis respectively. In contrast, alemtuzumab was a poor activator of CDC, even in the AIDS-NHL cell lines expressing high amounts of CD52, leading to a lysis of only 1-30%, whereas it was at least as strong as rituximab in inducing ADCC of the same lines (up to 30% specific lysis). Altogether, these data offer a first in vitro rationale supporting the therapeutic use of rituximab for CD20-positive AIDS-NHL.