Deletion of CD4 and CD8 Coreceptors Permits Generation of αβT Cells that Recognize Antigens Independently of the MHC

108

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αβ T cell receptors (TCRs) are genetically restricted to corecognize peptide antigens bound to self-major histocompatibility complex (pMHC) molecules; however, the basis for this MHC specificity remains unclear. Despite the current dogma, evaluation of the TCR-pMHC-I structural database shows that the nongermline-encoded complementaritydetermining region (CDR)-3 loops often contact the MHC-I, and the germline-encoded CDR1 and -2 loops frequently participate in peptide-mediated interactions. Nevertheless, different TCRs adopt a roughly conserved docking mode over the pMHC-I, in which three MHC-I residues (65, 69, and 155) are invariably contacted by the TCR in one way or another. Nonetheless, the impact of mutations at these three positions, either individually or together, was not uniformly detrimental to TCR recognition of pHLA-B*0801 or pHLA-B*3508. Moreover, when TCR-pMHC-I recognition was impaired, this could be partially restored by expression of the CD8 coreceptor. The structure of a TCR-pMHC-I complex in which these three (65, 69, and 155) MHC-I positions were all mutated resulted in shifting of the TCR footprint relative to the cognate complex and formation of compensatory interactions. Collectively, our findings reveal the inherent adaptability of the TCR in maintaining peptide recognition while accommodating changes to the central docking site on the pMHC-I.MHC restriction | T cell recognition | structural immunology | immunogenetics D uring thymic selection αβ T cell receptors (TCRs) are selected for weak reactivity with one or more self-peptides in complex with self-MHC (major histocompatibility complex), resulting in mature T cells being restricted to recognize processed peptides only when they are presented by self-MHC molecules (1). The exquisite specificity of TCR-pMHC binding must address the highly polymorphic nature of the MHC and variable peptide cargo. The antigen-recognition site of the TCR is made up of six complementarity-determining regions (CDRs), three each from the α and β chains (2). Currently, it is postulated that the CDR1 and 2 loops underpin the specificity or "bias" toward the MHC, whereas the CDR3 loops recognize the diverse range of bound peptides (3-5). Recent studies support the view that the T cell repertoire is intrinsically biased toward the MHC through contacts mediated by conserved TCR Vα and Vβ residues (6). However, the factors governing MHC restriction, a central paradigm of antigen-specific T cell immunity, remain unclear.Presumably as a consequence of the polymorphic pMHC landscape, and the inherent diversity of the responding T cell repertoire, structural studies have shown that the TCR engages the pMHC-I and pMHC-II surfaces in a range of different docking modes (2). Nevertheless, conserved pairwise interactions between closely related Vβ8.2 + TCRs and pMHC-II molecules were identified, leading to the concept that there are defined interaction "motifs" or "codons" between the Vα and/or Vβ domains of a TCR and a given MHC allotype (3,(6)(7)(8). Central to the "int...

supporting

confidence: 83%

Hard wiring of T cell receptor specificity for the major histocompatibility complex is underpinned by TCR adaptability

Burrows

Chen

Archbold

et al. 2010

108

122

“…Most dramatically, one laboratory constructed a mouse lacking MHCI, MHCII, CD4, and CD8 and introduced mutations to uncouple essential downstream TCR-signaling molecules from essential interactions (47)(48)(49). The mice developed a peripheral T-cell repertoire that contains T cells reactive to the surface protein CD155.…”

Section: Discussionmentioning

confidence: 99%

Class II major histocompatibility complex mutant mice to study the germ-line bias of T-cell antigen receptors

Silberman

Krovi

Tuttle

et al. 2016

The interaction of αβ T-cell antigen receptors (TCRs) with peptides bound to MHC molecules lies at the center of adaptive immunity. Whether TCRs have evolved to react with MHC or, instead, processes in the thymus involving coreceptors and other molecules select MHC-specific TCRs de novo from a random repertoire is a longstanding immunological question. Here, using nuclease-targeted mutagenesis, we address this question in vivo by generating three independent lines of knockin mice with single-amino acid mutations of conserved class II MHC amino acids that often are involved in interactions with the germ-line-encoded portions of TCRs. Although the TCR repertoire generated in these mutants is similar in size and diversity to that in WT mice, the evolutionary bias of TCRs for MHC is suggested by a shift and preferential use of some TCR subfamilies over others in mice expressing the mutant class II MHCs. Furthermore, T cells educated on these mutant MHC molecules are alloreactive to each other and to WT cells, and vice versa, suggesting strong functional differences among these repertoires. Taken together, these results highlight both the flexibility of thymic selection and the evolutionary bias of TCRs for MHC.T he genes for immunoglobulins (Igs), αβ T-cell receptors (TCRs), and antigen-presenting MHC proteins appeared at least 450 million years ago in the cartilaginous fish and are present in all modern vertebrates (1-3). The more primitive hagfish and lampreys lack these genes and have an adaptive immune system comprised of unrelated proteins (4). The main ligands for αβ TCRs are short peptides derived from self and foreign proteins, captured in a specialized groove of MHC class I (MHCI) and class II (MHCII) molecules and presented to T cells (5, 6). Functional Igs and TCRs are created by very similar recombination mechanisms involving fusion of V, J, and sometimes D gene segments with additional variations at the junctions to create an enormous potential repertoire of Igs and TCRs, suggesting a common, unknown evolutionary origin for these loci.These observations have raised several unanswered questions. For example,why did a separate TCR-rearranging gene system develop for lymphocytes recognizing peptide-MHC ligands? How did the extraordinarily polymorphic MHC genes stay functionally connected to TCR genes throughout 450 million years of evolution? One long-standing hypothesis has been that certain features of TCRs and MHC molecules are evolutionarily conserved to promote their interaction (7-10). Like Igs, the antigen-recognition portions of TCRs are partially encoded in the complementary determining region (CDR) CDR1 and CDR2 loops of germ-line TCR Vα (TRAV) and Vβ (TRBV) genes and are partially generated by somatic recombination processes that form the CDR3 loops. This initial repertoire is culled dramatically during T-cell development in the thymus. First, only those T cells whose TCRs have at least some minimal affinity for the selfpeptide-MHC molecules expressed in the thymus are positively selected for fu...

“…For instance, V␥9V␦2 ϩ T cells lack the coreceptors CD8 and CD4, which restrict antigen recognition in ␣␤ T cells to peptides that are presented in conjunction with MHC I and MHC II molecules, respectively. As a consequence, this allows selectivity for nonpeptide antigens without impairing signal strength (24) and, at the same time, releases the constraint for the need of conventional APCs for induction of V␥9V␦2 ϩ T cell responses. A second distinguishing feature of V␥9V␦2 ϩ T cells is their broad, polyclonal activation by a single class of nonpeptide ligands derived from microbes or stressed tissue cells with alternative or aberrant isoprenoid metabolism (7,8).…”

Section: Discussionmentioning

confidence: 99%

Cross-presenting human γδ T cells induce robust CD8 ⁺ αβ T cell responses

Brandes

Willimann

Bioley

et al. 2009

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