Type III bare lymphocyte syndrome: lack of HLA class II gene expression and reduction in HLA class I gene expression.

The bare lymphocyte syndrome (BLS) consists of an association between a combined immunodeficiency disease and a significantly reduced expression of either human histocompatibility leukocyte antigens (HLA) class I (HLA-A, -B, -C) or HLA class II (HLA-DP, -DQ, -DR) at the cell surface. BLS type III, the more frequent form of this syndrome, is characterized by impaired expression of both class I and class II antigens on patients' cells, in particular on leukocytes. We describe herein the demonstration that expression of HLA class I molecules was reduced by approximately half on Epstein-Barr virus-transformed B cells (LCL) derived from type III BLS patients. HLA class I mRNA level was also decreased to the same extent. Expression of HLA class I molecules was also very significantly reduced at the surface of these fibroblasts as was mRNA specific for HLA class I. Simultaneously, the expression of HLA-DR molecules on LCL was even more greatly decreased, and the expression of HLA-DQ antigens was virtually abolished. Molecular analysis demonstrated an absence of mRNA for the alpha- and beta-chains of HLA-DQ and HLA-DR in the patients' lymphocytes. In general, such patients present with an association of an absence of expression of HLA class II antigens and a significantly reduced expression of HLA class I antigens. The mechanism of this association is still uncertain.

Can one predict antigenic peptides for MHC class I-restricted cytotoxic T lymphocytes useful for vaccination?

The cytotoxic T-lymphocyte (CTL) response can be crucial for efficient immunological control of intracellular pathogens and the MHC class I-restricted CTL have a major role to play in this process. They recognize complexes associating antigen-derived peptides with MHC class I molecules expressed on infected target cells. The characterization of these antigenic peptides is thus a key issue for developing vaccines efficient in inducing specific CTL. Recently, by sequencing the whole set of self-peptides eluted from a given MHC class I molecule, Falk and colleagues have found that they have a homogeneous 8-10 residue length and contain allele-specific peptidic motifs with two conservative dominant anchor residues. The existence of consensus motifs opens the way for a strategy to predict the MHC class I-restricted T-cell epitopes and here we discuss such an approach using hen egg lysozyme (HEL) as an antigenic model. Two HEL peptides corresponding to allele-specific motifs were found, HEL(49-56) and HEL(70-78) peptides, which can associate with MHC class I H-2Kb and H-2Db molecules, respectively. The HEL peptide HEL(70-78) was found to be able to induce HEL-specific CTL in H-2b mice also. Moreover, using an empiricial approach, we have also characterized the N-terminal HEL(1-17) peptide as an immunodominant antigenic peptide in the H-2k haplotype. This peptide presented by H-2Kk molecules neither contained the corresponding allele-specific binding motif nor fitted the expected 8-10 residue length.(ABSTRACT TRUNCATED AT 250 WORDS)

High efficiency of endogenous antigen presentation by MHC class II molecules.

MHC class II molecules are involved in the presentation of both exogenous and endogenous antigens to CD4 T cells. Using the trans-membrane hemagglutinin (HA) from measles virus and the secreted hen egg lysozyme (HEL) as antigen models, we have compared the efficiency of MHC class II presentation by naive antigen presenting cells (APCs) pulsed with exogenous antigen with that of their transfected counterparts synthesizing endogenous antigen. B cells expressing even a very low amount of trans-membrane HA were found to present endogenous HA to I-Ed restricted T cell hybridomas with a high efficiency whereas their naive counterparts required to be pulsed with a comparatively high amount of exogenous HA. Similarly, MHC class II presentation of endogenous secreted HEL was found to be much more efficient when compared with that of exogenous HEL. Biochemical studies did not reveal any enhanced intracellular degradation of endogenous HEL. As expected, HEL was released in the surrounding medium within < 1 h. MHC class II presentation of endogenous HEL could not be explained by re-uptake by bystander APCs of HEL secreted in the surrounding medium. No sensitization of naive APCs could be observed either when co-cultured with HEL secreting cells or when cultured for 10 days with a sub-threshold amount of exogenous HEL. At the cell surface, I-Ed molecules immunoprecipitated from HEL secreting cells were found to be slightly enriched in SDS-resistant forms. These data raised the question of how peptides derived from endogenous transmembrane and secreted antigens can so efficiently reach an MHC class II loading compartment.

Cytosolic targeting of hen egg lysozyme gives rise to a short-lived protein presented by class I but not class II major histocompatibility complex molecules.

A way to study the role of intracellular trafficking of an antigen in its presentation to T cells is to target the antigen to various cell compartments of the antigen-presenting cells (APC) and compare the nature of the complexes associating major histocompatibility complex (MHC) molecules and antigenic peptides, expressed on the cell surface. MHC class I+ and MHC class II+ mouse L fibroblasts secreting hen egg lysozyme (HELs cells) or expressing HEL in their cytosol (HELc cells) were obtained after transfection with HEL cDNA and signal sequence-deleted HEL cDNA, respectively. HEL was evidenced in both HELs- and HELc-transfected cells and the former type of transfectant secreted a large amount of HEL. However, HEL produced in the cytosol exhibited a short half-life of less than 5 min. HEL-derived peptides could not be shown biochemically either in HELc- nor in HELs-transfected cells. We then studied the capacity of these cells to present HEL to HEL-specific class I- and class II-restricted T cells. Both cell types could be recognized by the HEL-specific MHC class I-restricted CTL clones. In contrast, MHC class II-HEL peptide complexes, recognized by HEL-specific helper T cell hybridomas, could be detected on MHC class II+ HELs- but not HELc-transfected cells. In vivo experiments showed, however, that HELc-transfected cells could provide host APC with HELc-derived peptides able to associate with MHC class II molecules. This was inferred from the capacity of MHC class II-HELc-transfected cells, unable by themselves to elicit any anti-HEL antibody response, to prime syngeneic and allogeneic mice against HEL. The priming was revealed by the induction of an antibody response after a boost with an amount of HEL unable itself to elicit an antibody response.