Antigen-binding receptors on T cells from long-term MLR. evidence of binding sites for allogeneic and self-MHC products

Thymic commitment of regulatory T cells is a pathway of TCR-dependent selection that isolates repertoires undergoing positive or negative selection

The seminal work of Le Douarin and colleagues (Ohki et al. 1987; Ohki et al. 1988; Salaun et al. 1990; Coutinho et al. 1993) first demonstrated that peripheral tissue-specific tolerance is centrally established in the thymus, by epithelial stromal cells (TEC). Subsequent experiments have shown that TEC-tolerance is dominant and mediated by CD4 regulatory T cells (Treg) that are generated intrathymically by recognition of antigens expressed on TECs (Modigliani et al. 1995; Modigliani et al. 1996a). From these and other observations, in 1996 Modigliani and colleagues derived a general model for the establishment and maintenance of natural tolerance (MM96) (Modigliani et al. 1996b), with two central propositions: (1) T cell receptor (TCR)-dependent sorting of emergent repertoires generates TEC-specific Treg displaying the highest TCR self-affinities below deletion thresholds, thus isolating repertoires undergoing positive and negative selection; (2) Treg are intrathymically committed (and activated) for a unique differentiative pathway with regulatory effector functions. The model explained the embryonic/perinatal time window of natural tolerance acquisition, by developmental programs determining (1) TCR multireactivity, (2) the cellular composition in the thymic stroma (relative abundance of epithelial vs hemopoietic cells), and (3) the dynamics of peripheral lymphocyte pools, built by accumulation of recent thymic emigrants (RTE) that remain recruitable to regulatory functions. We discuss here the MM96 in the light of recent results demonstrating the promiscuous expression of tissue-specific antigens by medullary TECs (Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004) and indicating that Treg represent a unique differentiative pathway (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003), which is adopted by CD4 T cells with high avidity for TEC-antigens (Bensinger et al. 2001; Jordan et al. 2001; Apostolou et al. 2002). In the likelihood that autoimmune diseases (AID) result from Treg deficits, some of which might have a thymic origin, we also speculate on therapeutic strategies aiming at selectively stimulating their de novo production or peripheral function, within recent findings on Treg responses to inflammation (Caramalho et al. 2003; Lopes-Carvalho et al., submitted, Caramalho et al., submitted). In short, the MM96 argued that natural tolerance is dominant, established and maintained by the activity of Treg, which are selected upon high-affinity recognition of self-ligands on TECs, and committed intrathymically to a unique differentiative pathway geared to anti-inflammatory and antiproliferative effector functions. By postulating the intrathymic deletion of self-reactivities on hemopoietic stromal cells (THC), together with the inability of peripheral resident lymphocytes to engage in the regulatory pathway, the MM96 simultaneously explained the maintenance of responsiveness to non-self in a context of suppression mediating dominant self-tolerance. The major difficulty of the MM96 is related to the apparent tissue specificity of Treg repertoires generated intrathymically. This difficulty has now been principally solved by the work of Hanahan, Kyewski and others (Jolicoeur et al. 1994; Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004), demonstrating the selective expression of a variety of tissue-specific antigens by TECs, in topological patterns that are compatible with the MM96, but difficult to conciliate with recessive tolerance models (Kappler et al. 1987; Kisielow et al. 1988). While the developmentally regulated multireactivity of TCR repertoires (Gavin and Bevan 1995), as well as the peripheral recruitment of Treg among RTE (Modigliani et al. 1996a) might add to this process, it would seem that the establishment of tissue-specific tolerance essentially stems from the "promiscuous expression of tissue antigens" by TEC. The findings of AID resulting from natural mutations (reviewed in Pitkanen and Peterson 2003) or the targeted inactivation (Anderson et al. 2002; Ramsey et al. 2002) of the AIRE transcription factor that regulates promiscuous gene expression on TECs support this conclusion. The observations on the correlation of natural or forced expression of the Foxp3 transcription factor in CD4 T cells with Treg phenotype and function (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003) provided support for the MM96 contention that Treg represent a unique differentiative pathway that is naturally established inside the thymus. Furthermore, Caton and colleagues (Jordan et al. 2001), as well as several other groups (Bensinger et al. 2001; Apostolou et al. 2002), have provided direct evidence for our postulate that Treg are selected among differentiating CD4 T cells with high affinity for ligands expressed on TECs (Modigliani et al. 1996b). Finally, the demonstration by Caramalho et al. that Treg express innate immunity receptors (Caramalho et al. 2003) and respond to pro-inflammatory signals and products of inflammation (Caramalho et al., submitted) brought about a new understanding on the peripheral regulation of Treg function. Together with the observation that Treg also respond to ongoing activities of "naïve/effector" T cells--possibly through the IL-2 produced in these conditions--these findings explain the participation of Treg in all immune responses (Onizuka et al. 1999; Shimizu et al. 1999; Annacker et al. 2001; Curotto de Lafaille et al. 2001; Almeida et al. 2002; Shevach 2002; Bach and Francois Bach 2003; Wood and Sakaguchi 2003; Mittrucker and Kaufmann 2004; Sakaguchi 2004), beyond their fundamental role in ensuring self-tolerance (e.g., Modigliani et al. 1996a; Shevach 2000; Hori et al. 2003; Sakaguchi 2004; Thompson and Powrie 2004). Thus, anti-inflammatory and anti-proliferative Treg are amplified by signals that promote or mediate inflammation and proliferation, accounting for the quality control of responses (Coutinho et al. 2001). In turn, such natural regulation of Treg by immune responses to non-self may well explain the alarming epidemiology of allergic and AID in wealthy societies (Wills-Karp et al. 2001; Bach 2002; Yazdanbakhsh et al. 2002), where a variety of childhood infections have become rare or absent. Thus, it is plausible that Treg were evolutionarily set by a given density of infectious agents in the environment. With hindsight, it is not too surprising that natural Treg performance falls once hygiene, vaccination, and antibiotics suddenly (i.e., 100 years) plunged infectious density to below some critical physiological threshold. As the immune system is not adapted to modern clean conditions of postnatal development, clinical immunologists must now deal with frequent Treg deficiencies (allergies and AID) for which they have no curative or rational treatments. It is essential, therefore, that basic immunologists concentrate on strategies to selectively stimulate the production, survival, and activity of this set of lymphocytes that is instrumental in preventing immune pathology. We have argued that the culprit of this inability of basic research to solve major clinical problems has been the self-righteousness of recessive tolerance champions, from Ehrlich to some of our contemporaries. It is ironical, however, that none of us--including the heretic opponents of horror autotoxicus--had understood that self-tolerance, or its robustness at least, is in part determined by the frequency and intensity of the responses to non-self. In the evolution of ideas on immunological tolerance, the time might be ripe for some kinds of synthesis. First, conventional theory reduced self-tolerance to negative selection and microbial defense to positive selection, while the MM96 solution was the precise opposite: positive selection of autoreactivities for self-tolerance (Treg) and negative selection (of Treg) for ridding responses. In contrast, it would now appear that positive and negative selection of autoreactive T cells are both necessary to establish either self-tolerance or competence to eliminate microbes, two processes that actually reinforce each other in the maintenance of self-integrity. Second, V-region recognition has generally been held responsible for specific discrimination between what should be either tolerated or eliminated from the organism. In contrast again, it would now seem that both processes of self-tolerance and microbial defense (self/non-self discrimination) also operate on the basis of evolutionarily ancient, germ-line-encoded innate, nonspecific receptors (Medzhitov and Janeway 2000) capable of a coarse level of self/non-self discrimination (Coutinho 1975). It could thus be interesting to revisit notions of cooperativity between V-regions and such mitogen receptors, both in single cell functions (Coutinho et al. 1974) and in the system's evolution (Coutinho 1975, 1980) as well. After all, major transitions in evolution were cooperative (Maynard-Smith and Szathmary 1995).

Fine specificity analysis of lactate dehydrogenase B-specific proliferating T cell clones: implications for the mechanism of alloreactivity

T cell clones of C57BL/6 origin which recognize porcine lactate dehydrogenase B (LDH-BP) together with the Ab molecule were characterized in terms of fine specificity for both LDH-B and self-major histocompatibility complex determinants. Using antigen-presenting cells from the Ab-mutant strain B6.C-H-2bm12 (bm12), three clonotypes could be distinguished: the first responds to LDH-BP + bm12, the second fails to respond and the third is alloreactive to bm12. The last clone exhibits additional alloreactivities to A molecules expressed in strains of H-2 haplotypes f, r, s, u, w6, w7, w16, w17 and w23. All three clonotypes give identical response patterns to a panel of 17 different dehydrogenase enzymes, and react to the same tryptic peptide of LDH-BP. Thus, these clones appear to recognize the same LDH-B epitope together with at least 3 different determinants of the Ab molecule. The data suggest that alloreactivity is more closely related to T cell specificity for self-major histocompatibility complex than to specificity for foreign antigen.

Receptor specificity of Ia-restricted T lymphoblasts activated against trinitrobenzene sulfonate-coupled spleen cells: recognition of distinct trinitrophenyl and Ia moieties

The receptor specificity of H-2-restricted T lymphoblasts activated against trinitrobenzene sulfonate (TNBS)-coupled spleen cells was examined using an antigen binding assay. A population of Lyt-1+,2-T lymphoblasts acquired syngeneic Ia determinants during 4 days of primary culture with hapten-coupled stimulator cells. Syngeneic Ia was not reexpressed after trypsin treatment of the T cells, but was found after incubation with soluble Ia shed from lipopolysaccharide-activated blasts. Self-Ia binding was specific in that Lyt-1+,2- but not Lyt-1-,2+ cells acquired the antigen, and in that self-Ia bound more effectively than allogeneic Ia material. To determine the relationship of self-Ia binding to the recognition of foreign antigen, the binding of trinitrophenyl (TNP)-coupled plasma membrane vesicles by TNP-specific T cells was studied. TNP-vesicle binding occurred via TNP and H-2(Ia) molecules on the vesicles in that binding was inhibited with antibodies against TNP or H-2(Ia) molecules but not non-major histocompatibility complex (e.g., Ly-6.2) molecules on the vesicles. Complete inhibition of TNP-vesicle binding by an Iak-restricted TNP-specific T-cell line occurred with soluble TNP-lysine, but not an unrelated hapten, N-iodoacetyl-N-(5-sulfonic-1-naphthyl)ethylenediamine (I-AED)-cysteine. Conversely, I-AED-cysteine, but not TNP-lysine, inhibited binding of I-AED-coupled B6 vesicles by B6 anti-I-AED T cells. Significant, but weak inhibition of TNP-vesicle binding by the anti-TNP line was observed with glycoprotein preparations containing partially purified self-Ia molecules. However, inhibition was specific for I-Ak molecules, in that inhibition was lost after removal of I-Ak molecules from the glycoprotein preparation, and very little inhibition occurred with soluble glycoproteins prepared from thymocytes which contained very little Ia material or from LPS blasts of an unrelated H-2 haplotype. These results suggest a recognition model in which TNP and Ia determinants are recognized by neighboring receptor combining sites.

T cell idiotypes recognizing self-major histocompatibility complex molecules: H-2 specificity, allotype linkage, and expression on functional T cell populations

An anti-idiotypic serum (antiserum 5936, B. Rubin et al., J. Exp. Med. 1979. 150: 307) was used to demonstrate receptor sites for self-major histocompatibility complex (MHC) antigens on T lymphocytes. The antiserum was raised by injecting rabbits tolerant to mouse Ig with a B6 anti-CBA (anti-H2k) alloantibody. It recognized a large proportion of T cells from H-2k strains carrying the b, c, d or e allele at the Igh-1 locus, but only a few T cells from H-2k strains with Igh-1 alleles a, f and j. Allotype linkage of the 5936 idiotype was also demonstrated by segregation analysis. The antiserum did not recognize either H-2k B cells or T cells from other H-2 haplotypes despite the presence of a permissive Igh-1 allele. The 5936 idiotype was found to be associated with several different antigen specificities, indicating that it is not located on the binding site for foreign antigen. Furthermore, the 5936 antiserum inhibited the binding of soluble Ik antigens by H-2k, Igh-1b, T cells, and, in the presence of complement, eliminated T cells responding to different antigens in an I-Ak-restricted fashion. Collectively, the data indicate that the structure bearing the 5936 idiotype is a receptor for I-Ak antigens, expressed by strains carrying the I-Ak allele and a permissive allele at the Igh-1 locus. The relevance of this finding to the MHC-restricted recognition of antigens by T cells is discussed.

Specificity of antigen binding by T cells; competition between soluble and Ia-associated antigen

Competitive antigen binding experiments were performed with purified T and B cells of C3H.SW (H-2b) mice. As antigen, (T,G)-A--L [poly-L(Tyr,Glu)-poly-DL-ALa-poly-L Lys] was used, both in an Ia-containing form, released by adherent cells (IAC-Puri and Lonai, Eur. J. Immunol. 1980. 10:273), and in regular solution. It was found that regular (T,G)-A-L did not compete with the binding of 125I-labeled-IAC-(T,G)-A--L even at a 10(4)-fold excess, whereas IAC-(T,G)-A--L inhibited binding at 10-fold excess. The specificity of (T,G)-A--L binding to high-responder T and B cells was compared by using related branched synthetic copolymers as competitors. B cells cross-reacted with (T,G)-A--L, (H,G)-A--L, (G)-A--L and (T,G)-Pro--L. In contrast, antigen binding C3H.SW T cells cross-reacted only with (T,G)-A--L and (Phe, G)-A--L to both of which they are Ir gene-controlled high responders. Evidence for the Ir gene control of IAC-binding T cells was obtained by showing that high X low responder F1 hybrid T cells preferentally bind IAC-(T,G)-A--L processed by processor cells deriving from the high-responder parental strain. These data are interpreted to suggest that T cells have high affinity for antigen plus self Ia complexes, whereas they have a much lower, if any, affinity for free antigen. It also follows from the results that the structure of the complex ligand may have a role in defining the specificity, H-2 restriction and Ir gene control of T cells.

Receptor specificity, functional characteristics, and cell-surface phenotype of a highly selected anti-I-Ab-specific, long-term T-cell line

A highly selected alloreactive T-cell line was developed by repeated restimulation of B10.D2/n lymph-node cells with irradiated C57BL/10Sn (B10) spleen cells in long-term MLC for up to 21/2 years. Continuous growth of the line requires restimulation every 2 to 4 weeks with fresh H-2b stimulator cells. The line proliferates strongly against H-2b but not against H-2d, H-2f, H-2q, H-2r, or H-2s stimulators. Analysis of recombinant mouse strains showed that the proliferative response is directed against I-Ab, but not Kb or Db determinants. During the growth period of the line, strong cross-reactivity with H-2p (B10.P) and weak cross-reactivity with H-2k strains (e.g., CBA/J and B10.BR) was observed. A clone with exquisite specificity for I-Ab, but with no cross-reactivity with H-2p or H-2k was isolated from the line; thus clonal heterogeneity of the line still exists despite the highly selective growth conditions.--The majority of T cells from the line or clone were shown to bind I-Ab but not Kb or Db determinants either spontaneously during restimulation with fresh B10 stimulator cells or via membrane vesicles expressing I-Ab determinants. No killing activity by the line in either specific or nonspecific cytolytic T-cell assays was observed nor was the T 145 glycoprotein, characteristic of killer T cells, detected.