Hla Expression Variants Null Soluble And Lowexpression Alleles

A number of HLA class I and class II genes have been described with mutations that prevent cell surface expression ("null" or "soluble") or cause reduced expression ("low")54,71 (Table 93.2). The greatest number of these expression variant alleles have been described for HLA-A* and -B", but have been observed for most, if not all, HLA loci. While most of these variants are rare, some are not and may often appear as part of extended haplotypes (e.g., DRB4*01030102N,16 DRB5*0108N,17 DRB5*0110N,18 A*24020102L,72 Cw*0409N73). Known expression variants can be typed by DNA methods, but serologic typing is useful for generic exclusion of such alleles.

The consequences of mismatches for null expression variants have not been reported. However, since the null alleles are generally considered not to produce surface HLA molecules, mismatching between an expressed and nonexpressed allele could result in allostimulation and an adverse transplant outcome.

The A'24020102L allele produces surface antigen sufficient to stimulate a T-cell response even though not detected by anti-A24 antibodies in serologic typ-ing.74- Severe acute GVHD in an A*24020102L patient transplanted with an A*24-negative donor was reported by Zanone-Ramseier and colleagues.75

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