Requirements for stimulation or anergy induction in alloreactive human T cell clones

The "two signal" concept for T cell activation is widely accepted. Signal 1 is commonly delivered via the antigen receptor, and signal 2 via accessory interactions. Delivery of both signals results in activation, signal 1 alone in induction of hyporesponsiveness. The nature of signal 1 in alloreactivity is not completely clear; most evidence suggests that a complex of foreign major histocompatibility complex molecules and their bound peptides is recognized. Interactions between B7 (CD80) ligand and CD28/CTLA-4 receptors are currently considered the most important sources of signal 2. Xenogeneic cells transfected with human genes provide useful stimulators for dissecting signals 1 and 2 in alloreactivity. We show here that the majority of DR-specific alloreactive T cell clones (TCC) fails to recognize Chinese hamster ovary (CHO) cells transfected with human DR, whether or not these are cotransfected with genes for CD80 or LFA-3. Stimulation was not observed even in the presence of a pool of peptides isolated by low pH release from B cell line (BCL)-derived DR molecules, or in the presence of synthetic peptides corresponding to the sequences of the three most commonly identified endogenous peptides. Lack of recognition was observed both in failure to stimulate proliferation and in failure to induce anergy. However, one TCC was identified which responded weakly to DR+ CHO cells, and for this clone, the presence of either CD80 or LFA-3 strongly enhanced proliferative responses. Anergy was not induced, even in the absence of CD80. Immobilized HLA-DR molecules purified from a BCL also failed to stimulate proliferation, but unlike the CHO transfectants, they did induce anergy. Stimulation with BCL also induced anergy if CD80-dependent interactions were blocked with soluble CTLA-4-Ig receptor. These results are consistent with the model that DR molecules expressed in the absence of appropriate peptide are simply not recognized by most alloreactive T cells, whereas DR molecules containing appropriate bound peptide are recognized as signal 1 and induce anergy. CTLA-4-Ig blocking confirms that CD80-dependent interactions can be important in preventing anergy induction, but that they are not always necessary is illustrated by the existence of a single clone which recognized DR molecules on CHO transfectants, giving very weak proliferation without CD80, and nonetheless no anergy induction.