Isolation of a human major histocompatibility complex class I gene encoding a nonubiquitous molecule expressed on activated lymphocytes

Soluble HLA class I antigen secretion by normal lymphocytes: relationship with cell activation and effect of interferon-gamma

HLA class I antigens are thought to be integral membrane proteins. However, soluble forms of these molecules have been detected. Our laboratory has recently shown that the predominant form of these soluble proteins present in human serum, spleen tissue and culture supernatant of activated lymphocytes exhibits molecular weight and structure similar to classical HLA class I antigens, but lacks HLA A or B polymorphic determinants. In the present study, the secretion of such soluble proteins by lymphocytes has been further explored. Phytohaemagglutinin-stimulated normal lymphocytes secrete considerable quantities of soluble HLA (sHLA) class I proteins. This secretion seems to be a general property of lymphocytes, since activation of T as well as B cells by appropriate mitogens equally induce sHLA I secretion. Lymphocytes require RNA and protein synthesis, but not DNA synthesis, for the secretion to occur. Kinetic studies reveal that maximal sHLA I secretion precedes the peak of DNA synthesis by 24 h. In vitro stimulation with antigens or alloantigens also provokes sHLA I secretion. Moreover, this phenomenon has also been detected for in vivo-activated lymphocytes, as enhanced spontaneous sHLA I secretion was observed in cultures of low-density blastic B and T cells, and of blood lymphocytes obtained from normal subjects who had received a booster immunization 5 days earlier. Interferon-gamma (IFN-gamma) increases the expression of membrane-bound class I antigens but does not induce any sHLA I secretion, suggesting that both molecules are under different regulatory mechanisms. Our results indicate that human lymphocytes, upon stimulation, actively secrete considerable amounts of a soluble form of these biologically relevant proteins.

HLA class I gene polymorphism in genetic hemochromatosis

The authors have studied HLA class I polymorphism in an attempt to identify a genomic marker of the hemochromatosis gene. Five enzymes were used (HindIII, EcoRI, EcoRV, PvuII, and HincII) in 47-106 unrelated patients and 71-91 controls. Both populations were HLA-typed. The relationship between the restriction fragments and HLA-A antigens was considered. Comparison of the two groups revealed a significantly high frequency of one 14 kb EcoRI fragment among the HLA-A3 hemochromatosis patients. The fragment hybridizes to an HLA-A specific probe but not to an HLA-B specific probe, and is associated with the hemochromatosis gene, which is itself associated with HLA-A3.

Activated human T lymphocytes express MHC class I heavy chains not associated with beta 2-microglobulin

We present here the molecular characterization of a new activation-induced surface structure on human T lymphocytes, termed LA45, with high homology (93% at protein level) to MHC class I molecules. Antigen modulation and sequential immunoprecipitation experiments revealed that LA45 and HLA class I proteins do not crossreact with the corresponding antibodies. Furthermore, LA45 is not associated with beta 2-m. On the other hand, we could show that the separation of HLA-A,B,C and beta 2m molecules, induced by SDS-denaturation, leads to a conformational change in the heavy chain in such a way that it becomes reactive with LA45. The 90/45 kD LA45 proteins thus appear to be non-beta 2m-associated MHC class I alpha chains that are selectively expressed by activated but not by resting human T lymphocytes.

Influence of recombinant interleukin-2 and alpha- and gamma-interferons on the induction of novel class I antigens

Previous results obtained in our laboratory showed that novel class I antigens, closely related to HLA-A (TM antigen related to HLA-A9 and GO antigen related to HLA-A24), were expressed on activated HLA-A9 or HLA-A24 peripheral blood lymphocytes (PBL), whatever activation factor was used [mitogenic stimulation (PHA, PWM, Con A), EBV transformation or alloactivation], but not on resting T and B lymphocytes. These antigens were also expressed on HLA-A9 or HLA-A24 common acute lymphoblastic leukaemias (cALL) and some 'immature' chronic lymphocytic leukaemias (B-CLL), but not in hairy cell leukaemias (HCL), most of the B-CLL, acute myeloblastic leukaemias (AML) and acute myelomonoblastic leukaemias (AMoL). In order to investigate the regulation mechanisms of these novel class I antigens, PBL and different leukaemic cell types were treated in vitro with recombinant IL-2 (rIL-2) and alpha and gamma-interferon (rIFN). Our results showed that without previous activation, but after culture with rIL-2, TM and GO antigens could be induced in HLA-A9 or HLA-A24 PBL and enhanced in HLA-A9 or HLA-A24 B-CLL cases, whereas no expression was found in HLA-A9 or HLA-A24 HCL, T-ALL, AML or AMoL. After culture with rINFs, the expression of TM and GO antigens could be induced on HLA-A9 PBL and all HLA-A9 leukaemic cell varieties. Our results support the hypothesis that the expression of class I antigens is induced at an early stage of the cell cycle.

HLA non-A,B,C class I genes: their structure and expression

Clearly, the human genome includes a group of genes closely related to but distinct from the HLA class I genes encoding the HLA-A, -B, and -C major transplantation antigens. These non-A,B,C class I genes, designated as HLA-E, HLA-F, and HLA-G, are on the short arm of chromosome 6 and part of the HLA class I gene family. Although the human HLA-E, -F, and -G genes have features in common with the murine Qa- and Tla-genes, e.g. little allelic polymorphism, their relationship to the murine Qa- and Tla-region genes remains unclear. It has been suggested that the nonclassical MHC class I molecules function as ligands for gamma-delta T lymphocytes. The speculation is supported by the recent reports of a murine Qa-1 restricted gamma-delta T cell hybridoma and recognition of a TL antigen by gamma delta T cell receptors. The amino acid sequences of the HLA-E, -F, and -G encoded proteins suggest that each protein is likely to fold three-dimensionally into a structure very similar to HLA-A2 and has a capability of presenting a bound peptide at the cell surface. In light of the possible role of bound peptide in the expression of a class I molecule at the cell surface, it is interesting to note that the HLA-E and HLA-F molecules, even in association with beta 2-microglobulin, could not be detected at the cell surface of a transfected B-LCL. In contrast, the HLA-G molecule was found at the surface of transfected B-LCLs. Both HLA-E and HLA-F are less similar in sequence to HLA-A,B,C than is HLA-G. One explanation would be that the HLA-E and -F molecules have a mutation such that they are no longer able to bind peptide. If the HLA-G molecule does function to present peptide to T lymphocytes, there are features unique to HLA-G that should impact on its ability to perform this function. Both the analysis of HLA-G RNA and protein in trophoblasts indicate that HLA-G, unlike HLA-A, -B, -C, is relatively nonpolymorphic. Since HLA-A,B,C polymorphism is thought to increase the number of different peptides that these molecules can bind, HLA-G is likely to be able to bind a relatively limited variety of peptides. HLA-G also differs from HLA-A, -B, and -C in that it seems to only be expressed by placental amniochorionic trophoblasts.(ABSTRACT TRUNCATED AT 400 WORDS)

Qa-12--a novel determinant of activated T and B lymphocytes

Investigation of the antigenic phenotype of activated lymphocytes using the broadly cross-reactive mAb 6.3.4 revealed two phenotypic alterations as compared with the resting lymphocytes. The Qa specificity Qa-m208 disappears after lectin activation of Qa-m208-positive T lymphocytes. Analysis of Q7 and Q9 transfectants expressing the Qa-2 polypeptides shows that Qa-m208 is an epitope of the Qa-2 antigen. Because the Qa-2 antigens are still expressed on T lymphoblasts which have lost Qa-m208, changes of the Qa-2 molecules occur and result in the loss of certain epitopes. The second phenotypic change that we observed is the appearance of a novel specificity, Qa-12. Its expression is induced by lymphocyte activation and it is expressed on lymphoblasts of both T and B cell origin. The presence of this novel non-ubiquitous antigenic specificity is determined by the Tla region.

Transfer and expression of three cloned human non-HLA-A,B,C class I major histocompatibility complex genes in mutant lymphoblastoid cells

The HLA-A, -B, and -C class I human histocompatibility antigens and the genes that encode them have been isolated and characterized. Apparently complete class I non-HLA-A, B, C genes have been identified on HindIII-generated 5.4-kilobase (kb), 6.0-kb, and 6.2-kb DNA fragments derived from lymphoblastoid cell line (LCL) 721. We studied the expressibility of these genes by subcloning them into the nonintegrating pHeBo vector and transferring the chimeric plasmids into mutant LCL 721.221. This mutant was derived from LCL 721 by means of immunoselections following gamma-ray mutagenesis that eliminated expressions of the HLA-A, -B, and -C alpha chains. The HLA-A, B, C-null phenotype of mutant 721.221 made it possible to monitor the expression of class I genes transferred into it by assaying cell surface binding of monoclonal antibodies BBM.1 and W6/32, which recognize beta 2-microglobulin and HLA class I alpha-chain epitopes, respectively. Increased binding of BBM.1 and W6/32 was clearly observed in transferents containing the class I gene of the 6.0-kb DNA fragment but not in transferents containing the class I genes of the 5.4- and 6.2-kb DNA fragments. However, one-dimensional gel electrophoresis of BBM.1 and W6/32 immunoprecipitates made with [35S]methionine-labeled cell lysates showed that transfer of each non-HLA-A, B, C class I gene into 721.221 resulted in the appearance of an alpha chain that coprecipitated with beta 2-microglobulin. The three previously unreported alpha chains differed from each other in size and were smaller than HLA-A, -B, and -C alpha chains. These observations clearly show that these three cloned, nonallelic, non-HLA-A, B, C class I genes encode alpha chains that can be expressed in human cells.