Gestión de partes de incidencias y problemas

1.7.1.1. HLA-DR locus.

HLA-DRB1 is the most polymorphic class II locus, and on some haplotypes is closely linked to a second functional DRB locus. DRB3 alleles are carried on DR3, 5 and w6 haplotypes, DRB4 alleles are carried on DR4, 7 and w9

haplotypes, while DRB5 alleles are found on DR2 haplotypes (Bell et al 1987). The DRB3 and DRB4 loci encode DRw52 and w53 respectively, although these molecules are expressed at 15 to 20 times lower levels than the DR antigens encoded by DRA1 and DRB1. (Kappes and Strominger 1988)

Comparison of DRB1 sequences reveal a very complex pattern of variation between alleles. The amino acid diversity is limited mainly to three hypervariable regions (HVR) at 9- 13, 25-38 and 67-74 (Reinsmoen and Bach 1990). DRB1

alleles can be closely related to each other with few amino acid differences between them, or alleles can share some HVRs, but resemble other alleles at other HVRs. It is rare to observe allele specific polymorphic regions.

The patchwork of related and unrelated regions between DRB1 alleles suggests that multiple gene conversion and/or

reciprocal cross-over events have occurred in addition to point mutations. There are numerous reports of putative gene conversion events having occurred in the DR region

1988, Bontrop et al 1988, Hurley et al 1988a). The

putative donor sites of the gene conversion events appear to be spread out over the HLA-D region, and include a DQB2 gene (Gregersen et al 1986), and a DRB3 gene (Gorski and Mach 1986).

1.7.1.2. HLA-D specificities

The DR locus is also thought to be largely responsible for the HLA-D specificities which are defined by the mixed lymphocyte reaction (MLR) and can split DR alleles into subtypes. Some HLA-D types can be associated with several DR genes.

HLA-D types reflect T cell defined epitopes on HLA-DR molecules. Reinsmoen and Bach (1990) compared the HLA-D

types Dw2 and Dw21 on serologically defined DR2 cells with DRB sequence variation. They concluded that the second and third variable regions explained most (but not all) Dw

variation. The importance of the second and third HVRs in providing epitopes for DR-directed T cells is confirmed by Termijtelen (1990). However the reactivity of some T cell clones cannot be explained by this model. Termijtelen

(1990) proposes that some T cell recognition epitopes could be masked by bound peptide in the antigen presenting site.

1.7.1.3. The HLA-DQ locus

DQA1 is the only class II A gene which exhibits significant polymorphism, although it is still less polymorphic than DQB1 (Kappes and Strominger, 1988). There is evidence for gene conversion events between DQA1 and DQA2, and amino acid replacements tend to be clustered between amino acid positions 45 and 80 in the first domain (Jonsson et al

1987). DQB1 has a similar pattern of polymorhpism to DRB1 with first domain HVRs at amino acid positions 26 - 37, 52

- 57 and 70 - 74 (Kappes and Strominger 1988). The DQA1 and DQB1 are in strong linkage disequilibrium with DRB alleles (Ryder et al 1981)

1.7.1.4. The HLA-DP locus

The DPB1 second exon contains almost all the HLA-DP allelic variation (Bugawan et al 1988, Bugawan et al 1989). When comparing the 19 recognised DPB1 allele sequences, the polymorphic sites are clustered into six HVRs, however no DPB1 allele exhibits allele specific sequence at any one hypervariable region. Rather, the alleles appear to be generated by the shuffling of a limited number of

polymorphic sequences at the hypervariable sites (Bugawan et al 1990).

The HVRs in DRB1 are believed to encode Dw specificities corresponding to the second and fourth HVRs in DPB1 second exon sequences, implying that perhaps not every variable

region in DPB1 encodes TCR recognised epitopes. For DP molecules, TCR defined epitopes are established using the primed lymphocyte test (PLT). If the amino acid variation at the second and fourth HVRs in DPB1 are compared with the six PLT defined specificities, each PLT defined specificity has a unigue pattern of amino acid seguence. However, at the second HVR the alleles DPB1*0201 and *0202 have

different amino acid sequences, yet PLT is unable to discriminate between these alleles (Nomenclature for factors of the HLA system, 1984) suggesting that some sequence variation may not be functionally significant.

When 10th International Histocompatibility Workshop cell lines were PLT typed (Yang et al 1989), some samples typed as PLT blank. That is, some cell lines expressed DP

molecules that were not recognised by the panel of primed lymphocytes, inferring that novel DP specificities were present. Thus there are more T cell defined DP

specificities than the six officially recognised.

Alleles that are shown not to be functionally important may still prove useful as genetic markers. So while not all alleles may be needed to be distinguished from each other in the context of tissue typing for transplantation, it is still important to be able to discriminate between all known alleles for disease, anthropological (Serjeantson

1.7.2. Inappropriate pairing of A and B chains

Extra class II polymorphism can arise from the improper pairing of the A and B chains of different isotypes

(intertypic); or of alleles at the same locus (intratypic).

In transgenic mouse experiments intertypic DRA/DQB

molecules have been observed but not DQA/DRB (Kappes and Strominger 1988) , so that it appears the efficiency of intertypic association depends on the loci involved. When expression levels of individual A and B genes were raised

in transgenic experiments, mixed isotype molecules were be observed containing the over-expressed chain (Sant and Germain 1989). It was postulated that the mechanism by which one A chain finds its corresponding B chain appears to be a competitive event; each respective A chain having a three to five fold preference for its corresponding B chain over B chains of other isotypes.

It is possible that significant levels of mixed isotype class II molecules are expressed after gamma interferon induction, which leads to relatively large concentrations of A and B chains being present in the cytosol, and it is conceivable that local concentrations of A and B chains may differ enough to allow intertypic associations. Such

associations may have a role in autoimmune disease as T cells would not recognise the mixed isotypic molecule. In human B cell lines, a DQB1/DRA1 molecule has been detected

The DQA locus is the only class II A locus which exhibits significant polymorphism. Thus it is possible that DQA and DQB chains from different chromosomes may associate forming

intratypic, or transcomplementary, HLA-DQ molecules. Intratypic DQ molecules have been observed in DQ

heterozygotes and may have a role in IDDM (Nepom et al 1987) and celiac disease (Sollid and Thorsby 1990).