CD52 Is Expressed on Human Mast Cells and Is a Therapeutic Target for the Anti-CD52 Monoclonal Antibody Campath-1H in Waldenstrom’s Macroglobulinemia and Mast Cell Disorders.

Campath-1H is a monoclonal antibody used in the treatment of CD52 expressing B-cell malignancies. Recently, we and others have observed that Campath-1H shows unusually high activity in patients with relapsed/refractory Waldenstrom’s macroglobulinemia (WM), a B-cell malignancy with excess bone marrow mast cells (BMMC). Importantly, as we have recently shown, BMMC appear to provide direct support for WM tumor cell growth (Tournilhac et al, JCO 2004 22:571S), and are therefore a potential therapeutic target in WM. We therefore examined BMMC from patients with WM and other mast cell disorders for cell surface expression of CD52, and targeting by Campath-1H in preclinical studies. Multicolor flow cytometric analysis demonstrated CD52 expression on BMMC (FceRI⁺, CD117⁺) from 13/15 WM; 2/2 systemic mastocytosis (SM) patients; 2/4 healthy donors; as well as on the LAD and HMC MC lines. Moreover, RT-PCR analysis confirmed CD52 expression in sorted BMMC from 6/7 WM, along with 6/6 healthy donors. Importantly, Campath-1H induced high levels of antibody dependent cell mediated cytotoxicity (ADCC) actvity against LAD mast cells using activated NK effector cells. No direct cytotoxicity or antiproliferative activity by Campath-1H on LAD cells was observed. These studies demonstrate that CD52 is widely expressed on human MC and provide support for the use of Campath-1H in the treatment of WM and other systemic mast cell disorders.