No abstract

The identity of the effector T cell population involved in contact hypersensitivity is still questionable with evidence promoting both CD4+ or CD8+ T cells. Previous experimental studies have relied on the in vivo depletion of T cell subsets using antibody, or the use of knock-out mice with deficiencies in either CD4+ or CD8+ T cell-mediated immunity. To address the role of the class I- and class II-mediated pathways of T cell activation in contact hypersensitivity responses in mice with an intact immune system, we utilized various trinitrophenyl-derivatized peptides, which bind specifically with H-2Kb (major histocompatibility complex class I) or H-2I-Ab (major histocompatibility complex class II). The subcutaneous injection of major histocompatibility complex class II-specific, but not of class I-binding, hapten-derivatized peptides in incomplete Freund's adjuvant induced specific, albeit low, contact hypersensitivity responsiveness to trinitrochlorobenzene. When bone-marrow-derived dendritic cells, however, were pulsed with the same peptides and administered intradermally, the opposite result was observed, namely that the class I binding peptides induced contact hypersensitivity responses similar to that observed after epicutaneous trinitrochlorobenzene application. In contrast, dendritic cells pulsed with major histocompatibility complex class II binding peptides did not reproducibly sensitize for contact hypersensitivity responses. Surprisingly, both immunization protocols efficiently induced CD8+ effector T cells. These results support the notion that CD8+ T cells are the dominant effector population mediating contact hypersensitivity responsiveness and that the CD4+ T cell subset only contributes little if at all.

The induction of contact sensitivity in mice by hapten reagents such as trinitrochlorobenzene (TNCB) involves the activation of class II major histocompatibility complex (MHC)-restricted, hapten-specific, CD4+ T cells. Reports from different laboratories have indicated that the relevant antigenic epitopes in such reactions might include hapten-conjugated, MHC class II-associated peptides. This study for the first time directly demonstrates that hapten-peptides account for the majority of determinants recognized by trinitrophenyl (TNP)-specific CD4+ T lymphocytes. The sequences of those TNP carrier peptides do not have to be related to mouse proteins. Thus, we show that TNP-modified peptides derived from mouse IgG, pigeon cytochrome c or staphylococcal nuclease known to bind to I-Ab or from lambda repressor with specificity to I-Ad as well as TNP-proteins such as bovine serum albumin, ovalbumin or keyhole limpet hemocyanin all create class II-restricted hapten determinants for a number of TNP-specific T cell clones and hybridomas. All of these cells were induced with cells modified by trinitrobenzene sulfonic acid (TNBS). In addition, we present arguments indicating that individual TNP-specific helper T cells may cross-react with different TNP-peptides bound to identical class II molecules. Chemical treatment of antigen-presenting cells with TNCB or TNBS may thus result in a limited number of particularly repetitive immunodominant hapten epitopes. Immunodominant epitopes were also indicated by an overrepresentation of the TCR elements V beta 2 and V alpha 10 in I-Ab/TNP-specific T cells. Most importantly, however, we demonstrate that TNP attached to lysine 97 in the staphylococcal nuclease peptide 93-105 (i.e. a clearly "non-self" sequence) is able to prime mice for subsequent elicitation of contact sensitivity by TNCB in the absence of foreign protein. We take this to indicate that those TNP-peptide determinants defined by us as immuno-dominant are responsible for the induction of contact sensitivity to haptens.

The elucidation of mechanisms underlying the recognition of haptens by class II MHC-restricted T cells is instrumental for the understanding of chemical- and drug-induced allergies. We have previously demonstrated that trinitrophenyl (TNP) peptides represent dominant antigenic epitopes for CD8+ and CD4+ mouse T cells triggered by chemically TNP-modified APC. Here, we report the characterization of TNP-specific, CD4+ mouse T cell lines and hybridomas that were induced in vivo and in vitro by defined hapten-conjugated peptides. These peptides, which we had previously shown to induce contact sensitivity to picryl chloride in vivo regardless of sequence homologies to mouse proteins, were found to activate carrier-independent TNP-specific T cells in vitro. We interpret these findings to support our view that carrier-independent T cells, reactive to particularly repetitive hapten epitopes, may play a crucial role in allergies to chemicals and drugs. In addition to carrier independence, one of our hybridomas (IT-H6/A11) exhibited a striking promiscuity of MHC restriction. Although absolutely dependent in its TNP reactivity on the presence of MHC class II molecules, the IT H6/A11 hybridoma completely ignored class II polymorphism and even reacted to TNP peptides presented on human DR molecules. Regarding hapten allergies in humans with a heterozygous situation for three types of class II molecules (DR, DP, and DQ), such promiscuous MHC restriction should lead to the presentation of even higher epitope densities to the respective T cell clones. Hybridoma IT-H6/A11, reacting to TNP independent of carrier peptide and of MHC haplotype, also allowed for an unusually systematic study of the minimal requirements for TNP recognition. Despite an almost complete ignorance of amino acid side chains on the carrier peptide, our data indicate a clearly position-specific interaction of hapten and TCR.