MMRI-23: A Potential Role in AML Antibody-Mediated Therapy.

Novel targets expressed on early AML precursor cell populations, with restricted expression on normal tissues, provide attractive options for antibody based therapeutics. We identified the CD300 family as Ig superfamily members with the capacity, through signaling motifs, to induce triggering and inhibitory signals to the leukocytes on which they are expressed. Here we report on an inhibitory member of this family, 35-L5, as a potential target for antibody-mediated therapy in AML. To assess its potential, we analyzed 35-L5 transcript and cell surface expression on normal blood cells, cell lines, and AML cells using quantitative RT-PCR and a monoclonal antibody (MMRI-23) made against a recombinant 35-L5 Ig fusion protein. RT-PCR showed its expression in normal PBMC was restricted to CD14⁺ monocytes, CD11c⁺ dendritic cells (myeloid DCs), monocyte derived dendritic cells (MoDC), with limited expression by B cells. MMRI-23 bound CD14⁺ monocytes, myeloid DC and MoDC, but not CD19⁺ B, CD4⁺ T, CD56⁺ NK cells and CD11c⁻ (plasmacytoid) DC. Interestingly, MMRI-23 binding varied between donors and 35-L5 expression was upregulated on mature MoDC, when compared to immature MoDC from the same donor (3 from 6 donors). However, MMRI-23 binding to PBMC remained stable when cultured with activation agents PMA and ionomycin, LPS, or PHA for 24 and 48 hours. 35-L5 transcript and surface protein expression was evident in the monocytic derived U937, leukemic derived HL60, erythroleukemic derived HEL, CML derived K562, and B lymphoblast derived Daudi cell lines, but was limited in the KMH2 and HDLM2 cell lines and absent in the Jurkat, Raji, L428 and Mann cell lines. Preliminary studies in AML using quantitative RT-PCR showed that 35-L5 transcript was increased compared to control normal CD14 cell expression (n=9). Furthermore, MMRI-23 bound to 6 of 8 AML samples of the FAB M1, M2 and M5 subtypes. MMRI-23 crosslinking induced neither cell apoptosis, nor cell differentiation and change of CD14, CD33, CD64, CD16, CD11b expression. Defining the role of 35-L5 on normal and AML progenitor cells is necessary to evaluate its potential as a possible target for antibody-mediated therapy. MMRI-23 may have potential for drug conjugated antibody-mediated therapy, but 35-L5’s inhibitory function may also be exploited to develop a novel mechanism for controlling myeloid leukemias.