Differential Antigenic Targets of Anti-Tumor Immune Response and Selective Immunity to Stem Cell Associated Group B SOX Proteins in Preneoplastic Versus Malignant Gammopathy.

Current information about specific antigenic targets of anti-tumor immunity in humans is largely restricted to patients with cancer. In order to gain insights into the nature of antigenic targets of host immune response in preneoplasia, we screened sera of patients with preneoplastic gammopathy (MGUS, n=28), multiple myeloma (MM, n=35), and smoldering MM (SMM, n=14) for the reactivity against a panel of 84 defined tumor antigens by a serum antibody detection array (SADA). Reactivity against 22 of the antigens in this panel was detected exclusively in the sera from MGUS, SMM or MM and not in sera from normal blood donors (n=27). Overall, reactivity to one or more of the antigens in this panel was detected in 40%, 85.7% and 54.2% of MGUS, SMM and MM patients, respectively. Interestingly, the pattern of antigenic reactivity differed between cohorts. Immune responses to certain antigens (e.g. cancer-testis antigen SSX-3 or methytransferase-3) were only detected in MM or SMM, respectively. On the other hand, immune responses to SRY-like -HMG-box2 (SOX2) protein were only seen in MGUS patients. In spite of cytogenetic similarities of tumor cells, the natural history of MGUS is markedly different from that of SMM or MM. SOX2 (a member of group B SOX family) is a transcription factor known to be expressed in embryonal neural and cancer stem cells and plays an important role in stem cell self renewal and differentiation. The observed differential reactivity to SOX2 was subsequently confirmed in separate ELISA based assays. Overall, anti-SOX2 IgG antibodies were detected in 11/55 (20%) of MGUS patients but in none of the SMM (n=25) and MM (n=23) patients tested (p=0.004). Light chains-specific ELISA revealed the polyclonal character of the humoral response thus ruling out SOX2 reactivity due to the monoclonal paraprotein itself. This is also supported by the detection of anti-SOX2 IgG in patients with non-IgG gammopathies. The absence of anti-SOX2 reactivity in SMM and MM could not be explained by global impairment of humoral immunity in MM, due to the detection of comparable reactivity to tetanus toxoid as a control antigen. SOX2 reactive IgG antibodies were present at high titers in all patients. For 8 of these patients enrolled on a prospective SWOG trial, follow up samples revealed that the pattern and extent of antibody reactivity remains stable over time. Together, these data provide evidence for broad anti-tumor humoral immune response in patients with MGUS and identify group B Sox proteins (SOX2) as a specific antigenic target in a significant proportion of patients with MGUS. The finding that antigenic targets of naturally occurring host immune response in preneoplasia may differ from those in cancer has broad implications for both early detection and immune prevention of human cancer. Protective immunity may depend not just on the presence of anti-tumor immune response, but also the “stemness” of the target population.