EVOLUTIONARY CONSERVATION OF CD4 AS A RECEPTOR FOR IL-16: PRELIMINARY EVIDENCE FROM XENOPUS LAEVIS
Greg Maniero, Stonehill College
Taylor Uccello, Gregory D. Maniero, Jacob Gillis. Stonehill College, 320 Washington Street, North Easton, MA 02357
IL-16 is a pro-inflammatory cytokine shown to attract and activate T cells by binding to CD4 outside of the MHC-binding domain. The role of CD4 as a receptor for IL-16 has been conserved throughout vertebrate evolution and fragments of recombinant human IL-16 (rhIL-16) can stimulate a variety of mammalian T cells.
Xenopus represents a well-established model for the study of the evolution and conservation of vertebrate immunity. The gene for CD4 is known in Xenopus but little characterization of this molecule or of CD4+ T cells, has been performed largely due to the absence of an anti-Xenopus CD4 antibody. The Xenopus CD4 gene lacks the sequence for the canonical FLXK MHC-binding site described on mammalian T cells but encodes an IL-16-binding site. Conversely, the gene for Xenopus IL-16 encodes a conserved CD4-binding sequence.
Xenopus lymphocytes labelled with rhIL-16 and separated on a magnetic column yielded a fraction of IL-16+ lymphocytes similar to that of CD8- cells. RT-PCR revealed that Xenopus lymphocytes cultured with rhIL-16 upregulate MHC class II mRNA, a phenomenon described in mammals. Peritoneal injection of rhIL-16 resulted in a substantial increase in lymphocytes in the coelom, suggesting that IL-16 attracts lymphocytes in frogs as it does in mammals. RhIL-16+ lymphocytes were detected, although at low levels, by immunohistochemistry and flow cytometry.
We conclude that Xenopus lymphocytes bind and are activated by IL-16, most likely through interactions with CD4. We intend to exploit this interaction to purify Xenopus CD4 and CD4+ cells and produce an anti-Xenopus CD4 antibody.