Lymphocyte function-associated antigen-1 (LFA-1; aL beta 2; CD11a/CD18) is a member of the integrin family of cell surface adhesion receptors, and binds to the widely-expressed intercellular adhesion molecules -1, -2, and -3 (ICAM-1, -2, -3). LFA-1-ICAM interactions play critical roles in T-lymphocyte immune responses, mediating firm adhesion to antigen-presenting cells and target cells and additionally providing co-stimulatory signals for T-cell activation (Diamond and Springer 1994; Gahmberg 1997). The importance of LFA-1-ICAM interactions to cells involved in an immune response is emphasized by the fact that individuals with a CD18 gene defect lack functional cell surface beta 2 chains and suffer from leukocyte adhesion deficiency, a potentially fatal disease characterized by recurrent pathogenic infections (Anderson and Springer 1987).

Navajos and genetically related populations have a 10-fold increased incidence of Haemophilus influenzae type b (Hib) disease compared with control populations. The Vkappa gene A2 is used to encode the majority of anti-Hib Abs, and these are the highest affinity anti-Hib Abs. Navajos carry a different allele of the A2 gene segment (A2b) that is defective in its ability to undergo V-J recombination. The A2b allele has only three nucleotide changes from the commonly occurring A2a allele, two of which could potentially affect its ability to recombine. In this study we used two independent in vitro assays to test whether the nucleotide change found in the A2b promoter and/or in the A2b recombination signal sequence (RSS) might be responsible for the decrease in recombination frequency observed in vivo. Using a luciferase reporter gene assay, we found no significant difference between A2a and A2b promoter activities. However, the competition recombination substrate assay showed a 4.5-fold reduction in the relative frequency of recombination of the A2b RSS compared with A2a. We show that this decreased frequency is due to a synergistic effect of the unique nucleotide change present in the heptamer of the A2b RSS and the shared nucleotide change present in the nonamer of both A2b and A2a. This in vitro relative frequency of rearrangement is not significantly different from that observed in vivo; therefore, the A2b RSS is probably the factor associated with the increased susceptibility to Hib disease among individuals carrying the A2b allele.