News and Publications

Regulation of Mature T-Cell Responses

S. Webb, M.E. Ozaki, M.A. Campbell,* L. Schultz, D.J. Redondo, T. Huynh, J.L. Ragazzo, L. Karlsson,** O. Winqvist,** P.A. Peterson**

* La Jolla Pharmaceutical Co., San Diego, CA
** R.W. Johnson Pharmaceutical Research Institute, San Diego, CA

T-cell activation leads to the development of distinct populations of effector cells with different types of activities. For CD4⁺ cells, these subpopulations of effector cells produce different patterns of cytokines that then influence the behavior of many other cell types, including lymphocytes, antigen-presenting cells (APCs), other leukocytes, endothelial cells, and epithelial cells. One of our major interests is to define the mechanisms involved in regulating both the activation of CD4⁺ cells and the type of effector cells generated during immune responses.

Although interactions between T-cell receptors (TCRs) and peptide bound to MHC molecules are critical in this respect, interactions between accessory molecules are also involved. We used 2 systems to define the role of particular accessory molecules in CD4⁺ T-cell responses. In one, Drosophila cells were transfected with murine MHC class II molecules and defined combinations of murine accessory molecules. These cells were used to present peptide antigens to class II--restricted CD4⁺ cells from TCR transgenic mice. The other approach involved use of knockout mice with targeted mutations disrupting the expression of defined accessory molecules.

Three main findings emerged from these studies. First, interactions between lymphocyte function--associated antigen 1 (LFA-1) and intracellular adhesion molecule 1 (ICAM-1) play a previously unappreciated role in the regulation of IL-4 production. Antigen stimulation of naive T cells with ICAM-1^- B7⁺ APCs induces strong production of IL-4. Coexpression of ICAM-1 on the B7⁺ APCs leads to proliferative responses associated with poor IL-4 production and enhanced IL-2 secretion. The reduction in IL-4 protein reflects poor induction of IL-4 mRNA.

Second, the accessory molecules engaged during T-cell activation regulate the expression and activity of other accessory molecules. For example, whereas ligation of CD28 promotes TCR-mediated induction of CD40L expression, coengagement of CD28 and LFA-1 markedly increases levels of CD40L. Although previous studies indicated an important role for CD40L in APCs and B-cell activation, we also found a critical role for this accessory molecule in the production of IFN-.

Third, brief exposure of naive CD4⁺ cells to peptide presented by B7^- ICAM-1⁺ APCs induces a profound refractory state reminiscent of the anergic responses described previously for T-cell clones. Thus, reexposure of these T cells to peptide presented by "professional" APCs induces proliferative responses only at high concentrations of peptide. The desensitization of these T cells is associated with downregulation of LFA-1 molecules. This finding suggests that regulation of expression of LFA-1 may be an important mechanism for regulating the threshold for T-cell responses. Currently, we are attempting to define key signaling events involved in mediating the functional outcome of these accessory molecule interactions.

In other studies, we are continuing to pursue a long-term interest in the activation of CD4⁺ T cells by superantigens. Although activation via superantigens is in many ways similar to activation via conventional peptide antigens, the signaling pathways involved have distinct differences. Whereas TCR recognition of conventional antigens triggered rapid tyrosine phosphorylation of both CD3 components and -chains, recognition of a superantigen encoded by mouse mammary tumor virus led to poor phosphorylation of -chains and limited phosphorylation of CD3 /epsilonbdy/-chains. Because -chain phosphorylation is considered a key component of early TCR signaling events leading to activation of downstream signaling pathways, the lack of hyperphosphorylation of -chains after stimulation with the superantigen suggests that alternative means may account for activation of these pathways. Because recognition of both superantigens and conventional antigens involves TCRs, the factors that determine the differences in early signaling events are of particular interest.

PUBLICATIONS

Hartwig, U.F., Karlsson, L., Peterson, P.A., Webb, S.R. CD40 and IL-4 regulate murine CD27L expression. J. Immunol. 159:6000, 1997.

Luksch, C.R., Winqvist, O., Ozaki, M.E., Karlsson, L., Jackson, M.R., Peterson, P.A., Webb, S.R. ICAM-1 inhibits IL-4 production by naive T cells. J. Exp. Med., in press.

O'Rourke, A., Webb, S.R. Crosstalk between T and B cells generates B APC able to induce inositol phosphate production in T cells responding to Mls^a superantigens. Eur. J. Immunol. 27:3253, 1997.