We report here that CD47 is expressed at high levels on a large subset of human ALL subtypes, is an independent prognostic predictor in ALL for survival and disease refractoriness in both mixed and high-risk ALL patients, and is a monoclonal antibody target for elimination of ALL blasts through macrophage-mediated phagocytosis. Together, these data suggest that ALL pathogenesis relies on mechanisms to evade innate immune recognition and that modulation of the innate immune recognition of leukemia cells may be a viable treatment modality.
CD47 is broadly expressed on hematopoietic cells and other normal tissues (23
), which could potentially lead to toxic effects with an anti-CD47 antibody therapy. Despite this expression, we have previously demonstrated that administration of anti-CD47 antibody to normal cells including human CD34+ BM precursors and PB cells in vitro
does not lead to phagocytic engulfment (5
). Also, administration of a blocking anti-mouse CD47 antibody to wild type mice causes minimal toxicity, principally an isolated neutropenia (5
). This lack of toxicity is likely not entirely due to CD47 expression level, as anti-CD47 antibody equally coated both normal and leukemia cells at the therapeutic dose administered. Alternatively, the phagocytosis stimulated by anti-CD47 antibody may be due to an imbalance of pro- and anti-phagocytic signals on leukemic blasts with expression of an as yet uncharacterized positive stimulus for phagocytosis present on leukemia cells but not normal cells. Such candidate stimuli include phosphatidylserine (24
), annexin-1 (25
), and calreticulin (26
) which are targets under active investigation.
Within the last several years, several cell surface proteins have been identified as therapeutic targets with monoclonal antibodies proceeding into early and late phase clinical trials. Most therapeutic antibodies in clinical development have been focused on B-ALL. One candidate is CD20, based on its expression in approximately 40 to 50% of B-ALL cases (reviewed in (27
)). Rituximab, an anti-CD20 antibody, initially approved for treatment of B cell lymphoma, has demonstrated a significant survival advantage when added to standard chemotherapy in some ALL clinical trials, particularly against the Burkitt’s subtype (28
). Although effective in adult CD20+ B-ALL, there is a paucity of clinical data on the efficacy of rituximab in pediatric ALL. In contrast to CD20, CD22 is expressed in a larger percentage of B-ALL cases, being present on greater than 90% of B-ALL. Epratuzumab, a humanized monoclonal anti-CD22 antibody, is currently being investigated with early clinical studies in relapsed ALL showing limited effect as a single agent (30
). However, anti-CD22 antibody-immunotoxin conjugates are being explored in Phase I trials (31
), since CD22 is reported to be rapidly internalized upon antibody binding (33
). In addition, antibodies and immunotoxins to other antigens including CD19 are currently being explored ((34
) (reviewed in (35
Perhaps the best success of targeted therapy has been observed in Ph+ B-ALL. Since its demonstration of efficacy in chronic myeloid leukemia, imatinib, an ABL tyrosine kinase inhibitor, has been utilized in Ph+ B-ALL with some success. As a single agent, imatinib can produce response rates of 20–30%; however, these response durations are short (36
). The combination of imatinib with chemotherapy has been more promising, with three-year overall survival rates of 55% in patients treated with imatinib+hyperCVAD compared to 15% for patients receiving hyperCVAD alone (37
In contrast to B-ALL, there are few antibody therapies being investigated for treatment of T-ALL. The most prominent antibody for T-ALL, alemtuzumab, is targeted at CD52, which is expressed on greater than 95% of normal lymphocytes and at higher levels on T compared to B lymphoblasts (38
). However, early phase clinical trials do not report a significant benefit as a single agent or in combination with chemotherapy for the treatment of relapsed T-ALL (39
In contrast to the targeted therapies developed for B-ALL and T-ALL, our data provides a strong pre-clinical rationale that an anti-CD47 antibody can be effective in eliminating both B- and T-ALL as well as high-risk ALL.