Sheep CD4⁺ αβ T cells express novel members of the T19 multigene family

The sheep T19 multigene family contains at least 50 genes which are thought to be expressed exclusively on γδ T cells, The archetypal T19 molecule (represented by a full-length cattle cDNA clone termed WC1) is thought to have a relative molecular mass of about 220,000 and to contain 11 scavenger receptor cysteine rich (SRCR) repeats and a long cytoplasmic tail, In this study, purified CD4⁺ and γδTCR⁺ sheep lymphocytes were examined by reverse transcriptase-polymerase chain reaction for the expression of T19 molecules. As expected, γδ T cells were found to express T19 molecules which closely resembled the archetypal form. However, CD4⁺ αβ T cells were found to express at least two different types of T19 molecules; one resembled the previously described T19 molecules of γδ T cells which possessed the archetypal WC1-like structure, but a novel type of T19 variant which lacked SRCR domains 10 and 11 was also found in CD4⁺ T cells but not γδ T cells. This novel molecule exhibited an unusual, incomplete SRCR repeat 9 joined directly to a hinge region. The transmembrane and cytoplasmic domains of this unusual T19 variant resembled the cattle T19 clone WC1, except that a complete exon within the cytoplasmic region was missing. These results, in contradistinction to existing serological data, suggest that expression of the T19 gene family is not confined to γδ T cells. Selected T19 genes are apparently expressed within CD4⁺ T cells and possibly other lymphocytes as well.