T cell receptor-independent CD2 signal transduction in FcR+ cells

Abstract

CD2 subserves both adhesion and signal transduction functions in T cells, thymocytes, and natural killer (NK) cells. In mature T lymphocytes, CD2-mediated signaling function apparently requires surface expression of T cell receptors (TCRs). In contrast, in CD2+ CD3- NK cells and thymocytes, signal transduction through CD2 is TCR independent. To resolve this paradox and characterize TCR-independent triggering mechanisms, we transfected a human CD2 cDNA into a murine mast cell line, C1.MC/57 (Fc∈RI+, FcγRII+, FcγRIII+), which is known to produce interleukin 6 (IL-6) as well as release histamine in response to crosslinking of Fc∈RI. In the CD2 transfectant, a combination of anti-T112 + anti-T113 monoclonal antibodies (mAbs) induced a rise in intracellular free calcium ([Ca2+]i), IL-6 production, and histamine release. As expected, no activation was mediated by the same mAbs in C1.MC/57. F(ab)′2 fragments of the activatory combination of anti-T112 + anti-T113 mAbs induced IL-6 in the CD2-transfected mast cells, demonstrating an Fcγ receptor ectodomain-independent triggering mechanism. In addition, either intact anti-T112 or anti-T113 IgG alone, which failed to induce [Ca2+]i mobilization in the transfectant, was able to induce IL-6 production. A mAb directed against both FcγRII (previously denoted as FcγRIIb) and FcγRIII (previously denoted as FcγRIIa) inhibits this induction. These results indicate that: (a) Ca2+ mobilization is not essential for IL-6 production; and (b) crosslinking of CD2 and Fcγ receptors via intact anti-CD2 IgG stimulates IL-6 production. Thus, CD2-mediated IL-6 production occurs by both Fc receptor ectodomain-independent as well as Fc receptor ectodomain-dependent mechanisms in these nonlymphoid cells. Northern blot analysis demonstrates that although the mast cells do not express CD3ζ or CD3η mRNA, they express Fc∈RIγ mRNA. The latter is a known component of FcγRIII as well as Fc∈RI, has significant homology to CD3ζ/η, and is thought to have a signal transduction function. In these mast cells, CD2 signaling machinery does not require CD3ζ/η and may be linked to the Fc∈RIγ subunit. We predict that this subunit or a related structure may confer a TCR-independent signal transduction pathway upon CD2 in CD3- NK cells, thymocytes, and certain B lymphocytes.