IDENTIFICATION OF A CANDIDATE CD5 HOMOLOG IN THE AMPHIBIAN XENOPUS-LAEVIS

We identified a novel T cell Ag in the South African clawed toad (Xeno pus laevis) by a mAb designated 2B1. This Ag is present in relatively high levels on most thymocytes, approximately 65% of splenocytes, 55% of PBL, and 65% of intestinal lymphocytes, but is rarely seen on IgM() B cells in any of these tissues. Lymphocytes bearing the 2B1 Ag prol iferate in response to stimulation with Con A or PHA, whereas the 2B1( -) lymphocytes are reactive to LPS. Biochemical analysis indicates tha t this Ag is a differentially phosphorylated glycoprotein of 71 to 82 kDa. The protein core of 64 kDa bears both N- and Q-linked carbohydrat e side chains. The amino-terminal protein sequence of the 2B1 Ag share s significant homology with both the macrophage scavenger receptor typ e I motif and the mammalian CD5/CD6 family. The biochemical characteri stics and cellular distribution of the 2B1 Ag suggest that it represen ts the CD5 homologue in X. laevis. While T cells constitutively expres s this highly conserved molecule, Xenopus B cells acquire the CD5 homo logue only when they are stimulated in the presence of T cells.