In vivo CTL immunity can be elicited by in vitro reconstituted MHC/peptide complex

Soluble MHC class I complexes for targeted immunotherapy

Major histocompatibility complexes (MHC) have been used for more than two decades in clinical and pre-clinical approaches of tumor immunotherapy. They have been proven efficient for detecting anti-tumor-specific T cells when utilized as soluble multimers, immobilized on cells or artificial structures such as artificial antigen-presenting cells (aAPC) and have been shown to generate effective anti-tumor responses. In this review we summarize the use of soluble MHC class I complexes in tumor vaccination studies, highlighting the different strategies and their contradicting results. In summary, we believe that soluble MHC class I molecules represent an exciting tool with great potential to impact the understanding and development of immunotherapeutic approaches on many levels from monitoring to treatment.

Tricks with tetramers: how to get the most from multimeric peptide-MHC

The development of fluorochrome-conjugated peptide-major histocompatibility complex (pMHC) multimers in conjunction with continuing advances in flow cytometry has transformed the study of antigen-specific T cells by enabling their visualization, enumeration, phenotypic characterization and isolation from ex vivo samples. Here, we bring together and discuss some of the 'tricks' that can be used to get the most out of pMHC multimers. These include: (1) simple procedures that can substantially enhance the staining intensity of cognate T cells with pMHC multimers; (2) the use of pMHC multimers to stain T cells with very-low-affinity T-cell receptor (TCR)/pMHC interactions, such as those that typically predominate in tumour-specific responses; and (3) the physical grading and clonotypic dissection of antigen-specific T cells based on the affinity of their cognate TCR using mutant pMHC multimers in conjunction with new approaches to the molecular analysis of TCR gene expression. We also examine how soluble pMHC can be used to examine T-cell activation, manipulate T-cell responses and study allogeneic and superantigen interactions with TCRs. Finally, we discuss the problems that arise with pMHC class II (pMHCII) multimers because of the low affinity of TCR/pMHCII interactions and lack of 'coreceptor help'.

Coreceptor CD8-driven modulation of T cell antigen receptor specificity

The CD8 coreceptor modulates the interaction between the T cell antigen receptor (TCR) and peptide-major histocompatibility class I (pMHCI). We present evidence that CD8 not only modifies the affinity of cognate TCR/pMHCI binding by altering both the association rate and the dissociation rate of the TCR/pMHCI interaction, but modulates the sensitivity (triggering threshold) of the TCR as well, by recruiting TCR/pMHCI complexes to membrane microdomains at a rate which depends on the affinity of MHCI/CD8 binding. Mathematical analysis of these modulatory effects indicates that a T cell can alter its functional avidity for its agonists by regulating CD8 expression, and can rearrange the relative potencies of each of its potential agonists. Thus we propose that a T cell can specifically increase its functional avidity for one agonist, while decreasing its functional avidity for other potential ligands. This focussing mechanism means that TCR degeneracy is inherently dynamic, allowing each TCR clonotype to have a wide range of agonists while avoiding autorecognition. The functional diversity of the TCR repertoire would therefore be greatly augmented by coreceptor-mediated ligand focussing.

Development of a rapid in vitro protein refolding assay which discriminates between peptide-bound and peptide-free forms of recombinant porcine major histocompatibility class I complex (SLA-I)

The extracellular domains of swine leukocyte antigen class I (SLA-I, major histocompatibility complex protein class I) were cloned and sequenced for two haplotypes (H4 and H7) which do not share any alleles based on serological typing, and which are the most important in Danish farmed pigs. The extracellular domain of SLA-I was connected to porcine beta2 microglobulin by glycine-rich linkers. The engineered single-chain proteins, consisting of fused SLA-I and beta2 microglobulin, were overexpressed as inclusion bodies in Escherichia coli. Also, variants were made of the single-chain proteins, by linking them through glycine-rich linkers to peptides representing T-cell epitopes from classical swine fever virus (CSFV) and foot-and-mouth disease virus (FMDV). An in vitro refold assay was developed, using a monoclonal anti-SLA antibody (PT85A) to gauge refolding. The single best-defined, SLA-I restricted porcine CD8(+) T-cell epitope currently known is a 9-residue peptide from the polyprotein of CSFV (J. Gen. Virol. 76 (1995) 3039). Based on results with the CSFV epitope and two porcine haplotypes (H4 and H7), the in vitro refold assay appeared able to discriminate between peptide-free and peptide-occupied forms of SLA-I. It remains to be seen whether the rapid and technically very simple in vitro refold assay described here will prove generally applicable for the screening of virus-derived peptides for SLA-I binding.

Antigen-specific modulation of an immune response by in vivo administration of soluble MHC class I tetramers

Soluble MHC/peptide tetramers that can directly bind the TCR allow the direct visualization and quantitation of Ag-specific T cells in vitro and in vivo. We used HY-D(b) tetramers to assess the numbers of HY-reactive CD8+ T cells in HYTCR-transgenic mice and in naive, wild-type C57BL/6 (B6) mice. As expected, tetramer staining showed the majority of T cells were male-specific CD8+ T cells in female HY-TCR mice. Staining of B6 mice showed a small population of male-specific CD8+ T cells in female mice. The effect of administration of soluble MHC class I tetramers on CD8+ T cell activation in vivo was unknown. Injection of HY-D(b) tetramer in vivo effectively primed female mice for a more rapid proliferative response to both HY peptide and male splenocytes. Furthermore, wild-type B6 female mice injected with a single dose of HY-D(b) tetramer rejected B6 male skin grafts more rapidly than female littermates treated with irrelevant tetramer. In contrast, multiple doses of HY-D(b) tetramer did not further decrease graft survival. Rather, female B6 mice injected with multiple doses of HY-D(b) tetramer rejected male skin grafts more slowly than mice primed with a single injection of tetramer or irradiated male spleen cells, suggesting clonal exhaustion or anergy. Our data highlight the ability of soluble MHC tetramers to identify scarce alloreactive T cell populations and the use of such tetramers to directly modulate an Ag-specific T cell response in vivo.

Molecular cloning and characterization of major histocompatibility complex class I cDNAs from woodchuck (Marmota monax)

The full-length cDNAs of woodchuck major histocompatibility complex (MHC) class I (MhcMamo-I or Mamo-I) genes were cloned by using cellular mRNA isolated from the peripheral blood mononuclear cells and liver tissues of woodchucks. DNA sequence analysis of Mamo-I cDNAs revealed that the coding regions of Mamo-I genes were about 1,080 bp long, encoding 359 amino acid residues. The deduced amino acid sequences of Mamo-I showed structural features like leader, alpha1, alpha2, alpha3, transmembrane and cytoplasmic domains, similar to their homologues in human and other mammals. Analysis of five full-length clones from unrelated woodchucks indicated a polymorphism within the alpha1 and alpha2 domains of Mamo-I heavy chain and a high conservation within the alpha3 and the transmembrane/cytoplasmic domains. Amino acid residues of the alpha2 and alpha3 domains that are supposed to be involved in the binding of MHC class I to CD8 molecule, were largely conserved in Mamo-I genes. Phylogenetic comparison of MHC class I genes of woodchuck and other mammals indicated a close evolutionary relationship between woodchuck and squirrel MHC class I. We tentatively named this region the locus A of Mamo-I genes (Mamo-A). Sequence analysis of 101 clones of alpha1 and alpha2 regions derived from 14 woodchucks revealed that at least 14 different alleles within Mamo-A exist. Among these 14 alleles identified so far, Mamo-A*01 and Mamo-A*09 were of the highest frequency of about 21.5% and 14.5%, respectively. Our results indicate that Mamo-I genes are of a similar molecular structure to those of human and other mammals.

Single Amino Acid Replacements in an Antigenic Peptide Are Sufficient to Alter the TCR Vβ Repertoire of the Responding CD8+ Cytotoxic Lymphocyte Population

Cytotoxic CD8+ T lymphocytes are activated upon the engagement of their Ag-specific receptors by MHC class I molecules loaded with peptides 8–11 amino acids long. T cell responses triggered by certain antigenic peptides are restricted to a limited number of TCR Vβ elements. The precise role of the peptide in causing this restricted TCR Vβ expansion in vivo remains unclear. To address this issue, we immunized C57BL/6 mice with the immunodominant peptide of the vesicular stomatitis virus (VSV) and several peptide variants carrying single substitutions at TCR-contact residues. We observed the expansion of a limited set of TCR Vβ elements responding to each peptide variant. To focus our analysis solely on the TCR β-chain, we created a transgenic mouse expressing exclusively the TCR α-chain from a VSV peptide-specific CD8+ T cell clone. These mice showed an even more restricted TCR Vβ usage consequent to peptide immunization. However, in both C57BL/6 and TCRα transgenic mice, single amino acid replacements in TCR-contact residues of the VSV peptide could alter the TCR Vβ usage of the responding CD8+ T lymphocytes. These results provide in vivo evidence for an interaction between the antigenic peptide and the germline-encoded complementarity-determining region-β loops that can influence the selection of the responding TCR repertoire. Furthermore, only replacements at residues near the C terminus of the peptide were able to alter the TCR Vβ usage, which is consistent with the notion that the TCR β-chain interacts in vivo preferentially with this region of the MHC/peptide complex.

Immunization with a LEAPS heteroconjugate containing a CTL epitope and a peptide from beta-2-microglobulin elicits a protective and DTH response to herpes simplex virus type 1

A ligand epitope antigen presentation system (LEAPS) heteroconjugate vaccine containing a CTL epitope (H1) from the HSV-1 immediate early protein ICP27 (322-332) and a peptide sequence (J) from beta-2-microglobulin (35-50) elicited protection from intraperitoneal viral challenge and promoted DTH responses. The H1 peptide and other H1 containing heteroconjugates did not elicit protection or DTH responses. Antibody to the H1 peptide could not be detected by ELISA following vaccination with peptide, heteroconjugate or natural infection. The LEAPS heteroconjugate appears to prime a Thl-like response which is subsequently boosted by infection. These studies show that attachment of the J peptide can make a CTL epitope into a vaccine which is immunogenic and promotes a protective Th1 type of response.