Characterization of a human antigen with sera from infertile patients.

We report the cDNA cloning and subsequent characterization of a novel antigen implicated in antibody-mediated human infertility. This antigen, designated AgX (unknown antigen), was identified originally by screening a human testis lambda gt11 cDNA expression library with infertile patients' sera known to contain anti-sperm antibodies. AgX cDNAs isolated from testis and placenta cDNA libraries (AgX-1 and AgX-2, respectively) differed by a 48-bp deletion in the open-reading frame (ORF). The AgX-1 and AgX-2 ORFs encoded putative peptide chains of 505 and 521 amino acids (approximately 55.5 and approximately 57.3 kDa), respectively. The AgX amino acid sequences contained consensus motifs indicative of NTP binding. However, computer homology searches did not identify any significant similarity with known sequences. Quantitative analysis using the reverse transcriptase-polymerase chain reaction (RT-PCR) indicated that the AgX-1 mRNA was fiftyfold more abundant than AgX-2 in the testis, while AgX-2 was more abundant than AgX-1 in somatic tissues. An anti-AgX peptide antiserum identified two AgX isoforms on Western blots of human tissue extracts. An abundant 56-kDa isoform was detected only in testis and sperm. These data suggest that the 56- and 58-kDa isoforms are AgX-1 and AgX-2, respectively. AgX was localized by immunofluorescence to the principal piece of the sperm tail. Therefore, antibodies against an AgX isoform may reduce fertility by affecting sperm function.