Philippa Marrack

Detection of Antigen-Specific T Cells with Multivalent Soluble Class II MHC Covalent Peptide Complexes

Multimeric soluble MHC class II molecules stably occupied with covalently attached peptides bind with appropriate specificity to T cell hybridomas and T cells from T cell receptor transgenic mice. There is a direct correlation between soluble T cell receptor affinity for monomeric MHC/peptide and level of binding of multimeric MHC/peptide to T cells. While binding of the multimeric MHC/peptide complex is proportional to T cell receptor affinity and expression level, there is little influence of T cell CD4.

Host DNA released in response to aluminum adjuvant enhances MHC class II-mediated antigen presentation and prolongs CD4 T-cell interactions with dendritic cells.

Significance Alum has been used to improve the efficacy of vaccines since the 1930s. Here we show that alum acts in part via host DNA to increase the interaction time between T cells and APCs. Many vaccines include aluminum salts (alum) as adjuvants despite little knowledge of alum’s functions. Host DNA rapidly coats injected alum. Here, we further investigated the mechanism of alum and DNA’s adjuvant function. Our data show that DNase coinjection reduces CD4 T-cell priming by i.m. injected antigen + alum. This effect is partially replicated in mice lacking stimulator of IFN genes, a mediator of cellular responses to cytoplasmic DNA. Others have shown that DNase treatment impairs dendritic cell (DC) migration from the peritoneal cavity to the draining lymph node in mice immunized i.p. with alum. However, our data show that DNase does not affect accumulation of, or expression of costimulatory proteins on, antigen-loaded DCs in lymph nodes draining injected muscles, the site by which most human vaccines are administered. DNase does inhibit prolonged T-cell–DC conjugate formation and antigen presentation between antigen-positive DCs and antigen-specific CD4 T cells following i.m. injection. Thus, from the muscle, an immunization site that does not require host DNA to promote migration of inflammatory DCs, alum acts as an adjuvant by introducing host DNA into the cytoplasm of antigen-bearing DCs, where it engages receptors that promote MHC class II presentation and better DC–T-cell interactions.

Expression of the Antigen-Specific MHC-Restricted T Cell Receptor

The genes and proteins of antigen-specific, MHC-restricted T cell receptors are now quite well understood, although little is currently known about the control of their expression. In this chapter we discuss various influences on the reactivity of peripheral T cells, and show that the germ line T cell receptor repertoire is probably read out randomly in immature, cortical thymocytes. Several factors affect expression on mature thymocytes and peripheral T cells. These include a selective process which occurs at the transition of immature to mature thymocytes, which may be selection for self-MHC restriction, and/or tolerance induction and is illustrated in the work presented here by blocking at this point with antireceptor antibodies, and by specific inhibition of maturation of immature thymocytes bearing a particular receptor allotype in an MHC-controlled fashion.

SOLUBLE EFFECTORS, MEDIATORS, AND FACTORS AND THEIR MECHANISMS OF ACTION

Publisher Summary This chapter comments on soluble effectors, mediators, and factors and their mechanisms of action. Cell-free mediators, produced by secretion, shedding, or cell disruption clearly can regulate the immune response. Whether or not some of these molecules generally function in the course of a normal immune response as cell-bound rather than diffusable mediators is a moot point, which, in most cases, cannot be resolved with the data at hand. Cell-free mediators can be divided into two categories: nonantigen-specific and antigen-specific or dependent. In general, nonantigen-binding mediators do not bear cell surface-related antigenic structures and it seems likely that these molecules are synthesized and secreted without going through a cell surface bound state. The antigen-binding group of mediators is more difficult to deal with. It includes molecules with apparent antigen specificity such as helper and suppressor factors in addition to those that although nonantigen specific, nevertheless bind to antigen in order to function, such as genetically restricted factor (GRF). The most difficult factors to fit into this unifying scheme are the helper factors, AEF, or the helper factors. Nonspecific mediators should act most efficiently when produced during direct donor cell-target cell contact.

IR GENES AND I REGION RESTRICTION

Publisher Summary This chapter attempts to answer several questions raised by the observation that T cells have apparent dual specificity for antigen and products of the MHC. One of the conclusions of these experiments has been that the thymus imprints its X region type on helper T cells, such that after maturation T cells can only recognize antigen in the context of the J region type(s) of the thymus. Similarly for Ir genes, if high-responder T cells mature in a thymus of low-responder type, they are phenotypically low responder, thus the thymus imprints its Ir type. During priming, the I region or Ir gene type of the antigen presenting macrophage is important. There is also some indication that the I region type of the macrophage is important before deliberate antigen administration, while the T cells are maturing in the periphery. In alternate experiments, this question can be examined by testing the ability of low responder T cells to become high responder if matured, primed, and challenged in high responder environments. Such experiments have been performed by others for cytotoxic T cells and have shown that in some cases, the Ir genes are expressed by the T cells themselves, suggesting a previously suspected T cell–cytotoxic T cell interaction. Experiments have been done using the helper T-cells of low responder genotypes for (TG)-A—L. Ir genes can be indeed special cases of I region restriction of helper T-cell activity, in which for some reason, for certain haplotypes, the dual recognition of particular antigens and particular J region products cannot occur.