Low Levels of Circulating CS1, a Newly Identified Multiple Myeloma (MM) Antigen for a Novel Humanized HuLuc63 Monoclonal Antibody, Is Detected in MM Patient Sera and Correlates with Active Disease.

A new humanized anti-CS1 mAb HuLuc63 significantly induces antibody-dependent cellular cytotoxicity (ADCC) against autologous CS1-expressing patient MM cells. Here we examined whether soluble CS1 is detectable in MM patient sera and affects HuLuc63-induced cytotoxicity against MM cells. Using a sandwich ELISA, CS1 was detected in 44% (23/52) of sera of 52 MM patients (1.2 to 35.3 ng/ml). Immunoprecipitation of sera from CS1 ELISA-positive and -negative MM patients using anti-CS1 mAbs (HuLuc63 and ChLuc90) followed by immunoblotting with HuLuc63 and 1G9 recognizing different epitopes of CS1, further confirmed two forms of CS1 only in CS1-positive MM patient sera. Detection of CS1 is associated with MM (p < 0.0001), since it is detected in MM patient sera, but not in MGUS (n=15) or in healthy donors (n=100). In additional sera of 199 MM patients with newly diagnosed MM, 90% (181/199) of MM patients have detectable CS1 (1 to >80 ng/ml). Median CS1 levels for International Stage System (ISS) I (n=100), II (n=53), and III (n=46) patients are 5.87, 9.37, and 8.37 ng/ml, respectively. The correlation between ISS and CS1 is moderate (spearman correlation coefficient = 0.197, p=0.005). Patients with ISS II/III had significantly higher CS1 levels compared with those with ISS I (median 9.0 vs. 5.9 ng/ml, p=0.006), suggesting a correlation of serum CS1 with active MM. Since patients with ISS II and III require therapy, while those with ISS I do not, these results suggest that circulating CS1 may indicate need for therapy and further support clinical investigation of anti-CS1 therapy using HuLuc63 in MM. Importantly, HuLuc63 has demonstrated significant dose-dependant activity, leading to reduced or eliminated human myeloma tumors (MM1S, OPM2, and L363) in xenograft mice models. Anti-MM activity of HuLuc63 in mice could be observed at sustained serum levels of >2 mg/ml, which is well above the levels of circulating CS1 protein observed in MM patients. Thus, it is proposed that serum CS1 will be an unlikely antibody sink in patients treated with optimal doses of HuLuc63. Since NK cells also express CS1, albeit at lower levels than MM cells, we next determined the effects of HuLuc63 on NK cell function. Pretreatment of NK cells with HuLuc63 (0.1 mg/ml) for 3 days did not alter NK-mediated ADCC against CS1-expressing MM cells (MM1S, H929, INA-6) via HuLuc63. Moreover, recombinant CS1-Fc at physiological serum levels (<200 ng/ml) did not significantly inhibit HuLuc63-induced ADCC against MM cells. Inhibition of HuLuc63-induced MM cell lysis by CS1-Fc was observed at higher concentrations (>200 ng/ml), confirming specific HuLuc63-CS1 binding. Together, these studies strongly support continued clinical investigation of HuLuc63 in MM.