The Spermatozoa Protein, SLLP1, Is a Novel Cancer-Testis Antigen in Hematologic Malignancies.
Authors: Wang Zhiqing , Zhang Yana , Mandal Arabinda , Zhang Jian , Giles Francis J. , Herr John C. , Lim Seah H. 
Published: April 2020

SLLP1 is a unique non-bacteriolytic c-lysozyme-like protein isolated from human spermatozoa. Antisera to SLLP1 blocks binding in the hamster egg penetration assay, suggesting that SLLP1 may be involved in sperm/egg adhesion. A recent study by dot blot analysis on RNA showed that SLLP1 was expressed only in the testis and in Burkitt lymphoma Raji cell line, suggesting that further studies are warranted to determine and characterize SLLP1 expression in tumor cells, in particular, fresh tumor specimens.

Using a pair of sequence-specific primers in RT-PCR, we found that SLLP1 transcripts could be detected in 5/8 myeloma cell lines, suggesting that SLLP1 may be expressed in tumor cells from some hematologic malignancies. When we applied the investigations to 52 primary hematologic malignant specimens, SLLP1 transcripts were detected in 6/17 myeloma, 4/14 CML, 3/11 CLL, 2/9 AML and 0/1 hairy cell leukemia. In contrast, SLLP1 transcripts were not detected in the peripheral blood (n=12) or bone marrow (n=3) from any healthy donors. The specificity of the PCR products was confirmed by either sequence analysis or restriction digest with Pvu II.

SLLP1 transcripts were translated into its corresponding protein in these tumor cells. Using tumor cell lysate in Western blot analysis, we detected SLLP1 protein in the myeloma cell lines and also in fresh malignant specimens, although positivities were only observed in specimens with high RT-PCR signals. All PCR-negative specimens were also negative in Western blot analysis. The specificity of the Western blot signals were confirmed in all cases by blocking assays with a high concentration of recombinant SLLP1 protein.

We next investigated the expression of SLLP1 in a large panel of normal tissues using RT-PCR and real time quantitative PCR. Both approaches showed that SLLP1 is a novel Cancer-Testis antigen in hematologic malignancies. SLLP1 was detected, at a level of 8206 copies/0.25 mcg total RNA, only in normal testis. We also found that the SLLP1 mRNA copy numbers in fresh hematologic tumor specimens were up to 2316 copies/0.25 mcg total RNA, i.e. more than 25% of the level found in normal testis. We cloned and generated SLLP1 recombinant protein from E coli and used the purified recombinant SLLP1 in an ELISA system to detect anti-SLLP1 antibodies. Using sera from 24 healthy donors and the mean + 2SD as the cut-off signal intensities, we found that high titer IgG antibodies directed at SLLP1 could be detected in the sera from 2/9 AML, 5/23 CLL, 6/27 CML and 14/51 myeloma patients. The specificity of the antibodies was confirmed in Western blot analysis. Probably due to the decreased sensitivity of the detection system in Western blot analysis, only 1/2 AML, 3/5 CLL, 4/6 CML and 7/14 myeloma SLLP1 antibody+ sera produced a signal in the Western blot analysis. Interesting, IgG2 was by far the commonest SLLP1 antibodies in these patients. There was a good correlation between SLLP1 gene expression and immune responses.

In summary, SLLP1 is a novel CT antigen in hematologic malignancies and is capable of eliciting B-cell immune responses in vivo in cancer-bearing patients. Our results support SLLP1 as a protein target appropriate for further in vitro study to define its suitability for immunotherapy.