Human C D l

Human CDl is a family of 5 non-polymorphic genes, that have been divided into two groups (Park and Bendelac, 2000). Firstly, CD la, CD lb, CD lc and CD le genes are present in humans but not in rats and mice. The second group is represented by CD l-d and is present in rats and mice. CDl molecules only show 30 % homology to class la genes but still associate with p2m and have an immune function. They present microbial and self-lipid antigens to T cells. It as been suggested that the CDl binding groove would accommodate the lipid tails of the antigen (Zeng et al., 1997). CDl isotype of the first group are mainly expressed on antigen presenting cells. CD l molecules have separate pathways for antigen presentation and do not follow the same pathways as class I molecules (Sugita et al., 1999). A model has been established in which CDl isotypes have evolved to target different endosomal compartments, where they would bind different lipids. It has been shown that on injection o f analogues of mycobacterial hexose-1 -phosphoisoprenoids there was T cell proliferation in patients infected with Mycobacterium tuberculosis (Moody et al., 2000).

1.7.4 H2-M3

H2-M3 is a mouse non-classical class I molecule which was first associated with the presentation of a maternally transmitted antigen (Mta). The Mta was identified as an N- formylated peptide derived from a subunit of NADH dehydrogenase (Loveland et al., 1990). H2-M3 is now known to specifically present N-formylated peptides derived from mitochondrial and prokaryotic sources (Shawar et al., 1991). N-formylated peptide, able to bind H2-M3 have been identified as part o f the proteins generated by the intracellular bacteria Listeria monocytogenes. H2-M3 has a much higher affinity for N- formylated peptide compared with non-formylated ones. Thus, it seems that H2-M3 has evolved to bind N-formylated peptides because only prokaryotes and mitochondria initiate protein synthesis with an N-formylated methionine. H2-M3 is only expressed at very low levels on the cell surface of B cells (Chiu et al., 1999). Cell surface expression is not induced by culture of B cells at low temperature, but can be induced by the presence of N-formylated peptide (Chiu et al., 1999). Addition of specific N-formylated peptides results in the detection of H2-M3 at the surface of B cells, T cells, macrophages and dendrite cells extracted from the spleen. However, this result was only true for some peptides such as LemA and Fr38. It seems that the cell surface expression of H2-M3 upon activation by N-formylated peptide LemA and Fr38 is TAP dependant (Chiu et al., 1999). In contrast with these results, the protection against Listeria monocytogenes by different H2-M3 restricted CD8+ T cells clone does not seem to be totally TAP dependent.

1.7.5 HLA-F

HLA-F was cloned and characterised by Daniel Geraghty (Geraghty et al., 1990). This gene is located on chromosome 6 at the telomeric end of the human MHC. Only limited coding polymorphism has been identified (Geraghty et al., 1990; Kunishima et al., 1999; Lury, Epstein and Holmes, 1990) and a HLA-F null allele has been reported in the Japanese population (Uchigiri et al., 1997). Across species, HLA-F is highly conserved in the human and chimpanzee differing by only three residues in the extracellular domain (Otting and Bontrop, 1993). It has been suggested that HLA-F may represent a basic ancestral gene from which the other human class I molecules have evolved (Shiina et al., 1999).

Sequence comparisons of HLA-F with other class I molecules suggest that it has a similar basic molecular structure to the other class I MHC molecules and their homologues. HLA-F conserves many features such as the cysteines required for the class I MHC disulphide bonds, consistent with a similar mature protein conformation. The HLA-F a3 domain is highly conserved, consistent with its involvement in the interaction with P2m. The HLA-F a l domain contains the conserved N-linked glycosylation site at the Asn 86. The position and length of the trans-membrane region are similar to those of the class la proteins, making it likely to be a membrane protein. However, analysis of the protein sequence in comparison with class la molecules also suggests that HLA-F may have more significant differences from the standard class la sequences than the other class Ib molecules, HLA-E and HLA-G. Among the 10 highly conserved residues pointing to the antigen binding site of class la molecules (Met 5- Tyr

7- Phe 22- Gly 26- Tyr 59- Tyr 84- Trp 143- Lys 146- Tyr 159-Tyr 171). HLA-F differs at 5 of these residues (positions 5, 22, 26, 84 and 146) compared to only two differences at positions 5 and 143 in HLA-E and one difference at position 143 in HLA-G (Geraghty et al., 1990).

Studies of the HLA-F transcript suggest a low level of expression, and a specific pattern o f expression. RNase protection assays detected higher levels o f HLA-F in B lymphoblastoid cell lines and resting T cells (Geraghty et al., 1990). HLA-F was also detected in foetal liver (Houlihan et al., 1992), and skin. HLA-F transcript was undetected in some T cell lines, fibroblasts, and adult liver (Geraghty et al., 1990; Houlihan et al., 1992; Lury, Epstein and Holmes, 1990). HLA-F seems to be the only class Ib promoter with a functional IFN-stimulated response element (ISRE) to IFN-y (Gobin et al., 1999), but NF-kB does not appear to induce transactivation of any class Ib gene including HLA-F (Gobin et al., 1998).

Shimizu et al have demonstrated that |32m could be immunoprecipitated with the HLA- F heavy chain from HLA-F transfected 721.221 cells. However, no cell surface expression was observed and they did not determine if HLA-F was associated with a peptide (Shimizu et al., 1988). These results differ from the results obtained with class la and HLA-G. It is now known that it is the lack o f peptide provided by the class I leader sequence which was responsible for the absence of HLA-E at the cell surface of 721.221 cells (Braud et al., 1998a). There are no published functional studies o f the HLA-F molecule and its function remains unknown.