Immunogenetics

The sequence of a feline class I pseudogene and its comparison with class I genes from other species is presented. The gene isolated is a pseudogene because of the presence of four stop codons and two frame shift mutations in the first-and second-domain encoding exons, as well as a mutation in a splice acceptor site in the third intron. By sequence comparison with the other class I sequences determined to date, theFLA pseudogene is most closely related to theHLA-A locus products (88% nucleotide identity).

The jawless vertebrates, represented by hagfish and lampreys, are the most advanced animals that apparently lack T cell and B cell receptors. As such, they offer unique opportunities for understanding the evolution of antigen receptors and variable (V)-type immunoglobulin (Ig)-like domains. In the present study, we describe four hagfish Ig superfamily (IgSF) members carrying V-type domains. None of them appeared to have direct counterparts in jawed vertebrates, indicating that many IgSF molecules have either evolved independently in jawed and jawless vertebrates or diverged to the extent that clear homology is no longer recognizable. One of the members encoded a molecule closely related to the previously described membrane protein designated novel ITAM (immunoreceptor tyrosine-based activation motif)-containing IgSF receptors (NICIR). We show here that NICIR is a polymorphic multigene family with at least three members and is expressed predominantly in peripheral blood leukocytes. Phylogenetic analysis indicates that among known proteins, NICIR is most closely related to the lamprey molecule recently proposed to be a potential ancestor of T cell receptors.

 Human immunoglobulin heavy chains are encoded by a highly complex locus, IGH, which encompasses many transcriptional units, including nine alternative constant region genes. Much of the constant region gene cluster was duplicated during hominoid evolution. In rodents, IGH is known to be regulated by multiple elements, including several enhancers 3’ of the alpha chain-encoding A constant region gene, CA, the last transcribed region. Sequences downstream of the two human CA genes, possibly containing homologous enhancer elements, have not yet been reported. By long-range mapping of genomic DNA, a cluster of unmethylated rare restriction sites, representing a potential CpG island, was previously reported downstream of each CA gene, close to the 3’ end of the duplicated region. Such potential CpG islands are candidates for additional IGH regulatory elements. We isolated bacteriophage clones containing these clusters of methylation-sensitive restriction sites, which lie within short CpG-rich stretches. Some of these sites showed tissue-specific methylation. 3’ of the unmethylated CpG-rich sequences, clones derived from the 5’ (telomeric) copy of the duplicated region, contained, in order, endogenous retroviral sequences, a processed ELK1 pseudogene, and the pseudogene IGHGP. Comparison with the presumed 3’ (centromeric) copy of the duplicated region showed that similarity was lost exactly at the end of the retroviral long terminal repeat sequences. These results imply that an endogenous retroviral integration was present immediately 3’ of IGH in the common hominoid ancestor and suggest models for the duplication of the region.

Although the human nonclassical class Ib major histocompatibility complex (Mhc) locus, HLA-G, is known to act as an immune suppressor in immune-privileged sites, little is currently known regarding participation of the rodent class Ib Mhc in similar pathways. Here, we investigated the expression properties of the mouse nonclassical Mhc H2-Q5 k gene, previously detected in tumors and tissues associated with pregnancy. We find that H2-Q5 k is alternatively spliced into multiple novel isoforms in a wide panel of C3H tissues. Unlike other known class I MHC, it is most highly transcribed in the brain, where the classical class Ia Mhc products are scarce. The truncated isoforms are selectively enriched in sites of immune privilege and are translated into cell surface proteins in neural crest-derived transfected cells. Furthermore, we present data supporting a model whereby Q5k isoforms serve an immune-protective role by donating their Qdm leader peptide to Qa-1, in a pathway homologous to the HLA-G leader fragment binding HLA-E and inhibiting CD94/NKG2A-positive cytotoxic cells. In addition, we report a previously unknown homolog of H2-Q5 k in the C57BL/6 mouse, which encodes Qdm, but is transcribed solely into noncanonical isoforms. Collectively, these studies demonstrate that H2-Q5 k , and its homologous class I-like H2 b gene may play tissue-specific roles in regulating immune surveillance.

Specific suppressor T cells (SSTC), primed in vivo with H-2 antigens, have been shown previously to inhibit DNA synthesis in the one-way, three-cell mixed lymphocyte reaction (MLR) provided that (a) the stimulator cells bear the priming H-2 antigens, and (b) the responder cells possessIC+S regions homologous to those of the SSTC. Anti-B10.A BlO.A(2R) SSTC (anti-Dd) and anti-A.AL A.TL SSTC (anti-Kk) are shown here to be able to inhibit the DNA synthesis triggered in MLR, not only by the corresponding antigens, Dd and Kk, respectively, but also by irrelevant, third-party H-2 and Mls products provided that the corresponding and third-party antigens are presented on the same stimulator cell. If stimulatorH-2 regions, whose products interact with SSTC and responders, are located on different stimulator cells within the particular MLR, SSTC activity is not elicited. Participation of cytotoxic T lymphocytes in DNA-synthesis suppression is ruled out. Direct contact or location of the inhibited responder cell very close to SSTC is considered to be required for the development of SSTC activity.

SH2D1A, the X-linked lymphoproliferative disease (XLP) gene, encodes a cytoplasmic protein that plays an essential role in controlling Epstein-Barr virus infection. It is expressed in T and NK cells, but not in B cells or in granulocytes. The promoter, the regulatory regions, as well as the mechanisms controlling its tissue-specific expression, are still unknown. We tested the hypothesis that DNA methylation might contribute to tissue-specific SH2D1A gene expression and analyzed the methylation status of 2,300 bp upstream of the ATG starting codon, the coding region and part of intron 1. By bisulfite sequencing and methylation-sensitive restriction enzyme digestion, we show that a differential methylation pattern of CpG-rich regions in the 5′ region and the adjacent exon 1 of the SH2D1A gene indeed correlates with the tissue-specific gene transcription.

The genes encoding three invariant components of the human T-cell antigen receptor, the CD3 δ, γ, and ε chains, are located on human chromosome 11 at band q23. We isolated cosmid clones containing the human CD3 δ and γ chain genes in vectors designed for rapid and efficient chromosome “walking”. The human CD3ε gene was located in the region immediately downstream of the CD3 δ and γ genes using synthetic oligonucleotide probes and the localization of this gene confirmed by DNA sequencing. Detailed restriction mapping of the CD3 locus demonstrated that all three CD3 subunits are encoded within 60 kb of DNA with the CD3ε gene located 26 kb downstream of the CD3 δ and γ genes. Analysis of genomic DNA on pulsed field gels using probes isolated from these cosmid clones defined a physical map of 750 kb spanning the CD3 locus on human chromosome 11g23. The CD3 genes thus comprise a multigene family encoding cell surface components important for transmembrane signaling on T lymphocytes. The arrangement of these genes suggest that they may share common regulatory elements for the control of gene expression during T-cell ontogeny.