A separate class of immunogenically potent molecules presented by MHC class II complexes and recognized by T cell receptors is termed 'superantigens' (White et al., 1989). The most distinct feature of superantigens is that a single superantigen can stimulate a large proportion of ap TCR-bearing T cells, for some reaching up to 30% of the entire T cell population. This hyperimmunogenicity is because the MHC class Il-superantigen complexes are recognized only by Vp (the variable region of P-chain), or in some cases Vy, of T cell receptor, while MHC class II molecules complexed with conventional antigenic peptides are contacted by all the variable elements of TCR (Va, Ja, vp. Dp and Jp). This results in a much higher specificity of T cells recognizing conventional antigens. Indeed, the frequency
of T cell responses to a peptide antigen ranges usually from 10"^ to 10"^. The fact that superantigens interact with specific regions of the TCR distinguishes them from polyclonal T cell mitogens, such as concanavalin A. Furthermore, superantigens bind the MHC molecule at sites distinct from the peptide-binding groove (Dellabona et al.,
1990) [and different staphylococcal toxins bind to distinct sites on an MHC class II molecule (Scholl et al., 1989)], they do not require intracellular processing (Fraser et al., 1989) and their recognition is only marginally restricted by the MHC allele. Interactions between MHC molecules and superantigens (Dellabona et ah, 1990), MHC molecules and Vp of TCR (Fleischer et ah, 1991), and superantigens and Vp (Pullen et ah, 1990; Gascoigne and Ames, 1991) have been demonstrated and shown to be involved in the generation of T cell responses. The sole binding of some superantigens to MHC class II molecules can induce transcriptional activation and increase in MHC-bearing cell adhesiveness (Trede et ah, 1991), and may synergize in lymphocyte proliferation (Fuleihan etal., 1991).
A.5.1. Classification and manifestation of superantigens.
Superantigens can be divided into to two categories: foreign and self. The former comprize many bacterial products including e.g. staphylococcal enterotoxins, causative agents of food poisoning and toxic shock syndrome, or streptococcal toxins, whose involvement in multiple autoimmune diseases and immunodeficiencies in humans have been suggested (Heber-Katz and Acha-Orbea, 1989). The latter are exemplified by endogenous antigens found in mice - minor lymphocyte-stimulating determinants (Mis; Festenstein, 1973). There are two major manifestations of self superantigens. Firstly, certain self-superantigens eliminate during T cell selection in thymus those T cells, which carry TCR VP chains specific for these superantigens. Secondly, they cause an extensive proliferation in a mixed-lymphocyte assay, i.e. they are seen as ’alloantigens' in mixed cultures of spleen cells isolated from mouse strains that carry identical MHC genotypes, but differ in their Mis loci.
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A.5.2. Retroviral superantigens.
Self-superantigens have recently acquired a virological dimension. Although the Mis loci are unlinked to MHC, they co-segregate with integrated genomes of endogenous retroviruses, murine mammary tumour viruses (MMTV; Frankel et al.,
1991; Woodland et al., 1991; Dyson et al., 1991). In addition, an exogenous MMTV encodes a maternally inherited superantigen (Marrack and Kappler, 1991). The superantigen was transmitted from an infected mother to her offspring after birth (presumably in the milk) and caused a deletion of Vpl4+ T cells in the progeny. Further analysis located the superantigen gene to an open reading frame in the 3' long terminal repeat (LTR ORF) of MMTV (Choi et al., 1991). This finding was
confirmed by depletion of Vpl4+T cells in mice transgenic for both the whole MMTV and for just the LTR ORF. LTR ORF comparison of different MMTV
variants located the vp specificity to the C-terminal of the gene product (Acha-Orbea
et al., 1991). Another and potentially more important finding was a demonstration of a Vp5“Specific superantigen expressed by a defective murine leukaemia virus, which induces an AIDS-like disease in mice (Hugin et al., 1991). B cell lymphomas
presenting this retroviral superantigen caused a VP5+ T cell proliferation, which was possible to block by gag p30-specific antibodies. Thus, superantigens may represent yet another mechanism for perturbation of the fine equilibrium of our immune system, potentially contributing to the development human AIDS.
A.5.3. What is the biological function of superantigens?
What is the function of superantigens that preserved them through the
evolution? There are probably 20 to 30 integration sites for MMTV in mice, these are relatively recent in evolution and may not have their counterparts in other mammals as e.g. no endogenous superantigens have yet been detected in humans. In mice. Mis could evolve to delete bacterial toxin-responsive T cells (Marrack et al., 1990). On the other hand, subtle changes in either the Mis loci or the respective Vp regions were found in wild mice, preventing the negative selection of Vp+ T cells during the maturation in thymus (Cazenave et al., 1990). In addition to negative selection, a role
of Mis in positive selection has been also observed (Benoist et al., 1989) as well as their ability to potentiate conventional antigen presentation (Janeway et al., 1983). This led to the suggestion of a co-ligant function for superantigens (Janeway et al.,
1989), whereby Mls-like structures would in special cases stabilize or orient the TCR- MHC interactions. Also, because bacterial superantigen stimulation does not require the CD4 molecule (Fleischer and Schrezenmeier, 1988; Sekaly et al., 1991), these superantigens may substitute for the CD4 function. These properties could be exploited for potentiating immune responses induced by vaccines. In any case, superantigens in mice exert a strong influence on the shaping of the T cell repertoire. Their true biological function and role in autoimmune and immunodeficiency
diseases, however, remains obscure.
A.6. Humoral immune responses.
Humoral immune responses defend an organism primarily against the extracellular phase of viral infections. They are mediated by antibodies, which circulate throughout the body in the lymph and in the blood serum. Antibodies are produced by B lymphocytes, which upon recognition of a foreign antigen differentiate into antibody-secreting plasma or memory B cells. For most antigens, these processes are assisted predominantly by helper Th2 cells (above). Once produced, antibodies can bind to a virtually unlimited number of surface structures on the virus and neutralize it by either preventing attachment to and/or penetration into the host cells, or opsonizing the virus for efficient phagocytosis by macrophages. In some instances, these processes may happen more efficiently in the presence of complement.
Antibodies recognize small antigenic determinants called epitopes, which can be continuous or discontinuous (Barlow et al, 1986). Discontinuous epitopes depend on the tertiary and quaternary protein structures and are always conformationally sensitive, i.e. are destroyed upon antigen dénaturation or proteolytic degradation. Continuous epitopes consist of a linear array of amino acids and are usually
2 8 by antigen-binding sites on antibodies, called paratopes. It is assumed that all epitope- paratope interfaces involve a surface area of approximately 700 Â2, as it was shown in the case of lysozyme (Amit et ah, 1986). From the known folding patterns of many globular proteins, this assumption implies, that no surface epitope region is likely to contain only a single continuous stretch of amino acid residues. Thus, 'continuous epitope', in fact, usually represents only part of a larger discontinuous determinant. It follows that linear peptide fragments are likely to have only a across-reactive
antigenicity with the native protein, from which they were derived. As discussed below, this raises some theoretical questions about peptide vaccines. Several
computer programmes have attempted to predict linear cross-reactive fragments from the primary protein structures (Barlow et al., 1986; Blundell et al., 1987). But, as it is with the prediction of T cell sites, these algorithms predicted well some 'continuous' epitopes, while they failed to identify others.