Semg 1 Expression in Early Chronic Lymphocytic Leukemia

SEMG 1 is major protein of semen coagulum shown to inhibit human sperm capacitation. It plays an important role in sperm clotting and is normally degraded into smaller fragments by prostate-specific antigen. The gene encoding SEMG 1 has been localized to the long arm of chromosome 20, a region of chromosome 20 that is frequently deleted in myeloproliferative diseases and myelodysplastic syndrome. We previously found SEMG 1 to be a Cancer-Testis (CT) antigen that was immunogenic in the cancer-bearing patients, supporting its potential as a target for tumor immunotherapy. CLL patients are generally immunosuppressed even before any therapy is given. The immunosuppression increases as the disease progresses and may prevent successful immunotherapy unless the therapeutic approach is used to patients with early stage disease (Stage 0 or I), even prior to use of any immunosuppressive chemotherapy. However, most CT antigens are expressed in low frequency in early stage cancer. Accordingly, we set out to determine the expression frequency, variant and pattern of expression of SEMG 1 in patients with early CLL.

Using a pair of sequence-specific primers in RT-PCR on total RNA derived from patients with early CLL, we found that SEMG 1 transcripts could be detected in 19/41 (46%) patients. The high frequency at which SEMG 1 is expressed in early CLL is in contrast to the generally low frequency of expression of other CT antigens in early malignancies. SEMG 1 expression not only occurred at the transcript level but also protein level, as determined by immunocytochemistry using SEMG 1 MoAbs. These results indicate that SEMG 1 could potentially be a suitable target for the design of tumor vaccine that could be applicable to a large proportion of patients with early CLL. Of the 19 patients expressing SEMG 1, eight expressed Zap 70 protein and 11 did not (p = 0.4; N.S.). Therefore, any SEMG 1-based tumor vaccine could be applicable even to patients with early poor risk CLL (as determined by Zap 70 expression). However, SEMG 1 expression, as determined by immunocytochemistry, within individual CLL patients was heterogeneous, suggesting that tumor vaccine targeting SEMG 1 may have to be administered with therapeutic agents that upregulate SEMG 1 expression to overcome the potential problem of tumor antigen heterogeneity that could prevent the success of the tumor vaccine.

Since SEMG 1 exists in two variants, we next determined which SEMG 1 variant was expressed in CLL cells. Using a primer pairs in PCR to amplify across the 180 bp transcript that is deleted in SEMG 1₄₃, we showed that in all cases of SEMG 1-positive CLL specimens, only the non-truncated transcripts were detected, indicating the expression of SEMG 1₅₀ variant in CLL.

Finally, applying diluted patient serum to an ELISA system, we found that high titers SEMG 1 IgG antibodies could be detected in six of these 41 patients but not in any of the 20 healthy donors. Leukemia cells from three of these patients expressed SEMG 1. These results, therefore, suggest that SEMG 1-reactive lymphocytes are present in the immune repertoire of patients with early CLL, irrespective of whether the leukemia cells express SEMG 1, and further support the feasibility of applying a SEMG 1-based tumor vaccine to these patients.

In conclusion, SEMG 1₅₀ is expressed by the leukemia cells in a high proportion of patients with early CLL, irrespective of the Zap 70 expression status. SEMG 1 may be an immunological target for immunotherapy of nearly half of early CLL patients, especially when SEMG 1-reactive lymphocytes are present in the immune repertoire of these patients.