A key question in the development of ABPA in CF and asthmatic patients is why do only 1–2% and 7–9% of this at risk population develop ABPA. Chauhan et al [3–5] observed that patients with asthma and CF who expressed HLA-DR2 and/or DR5 (and perhaps HLA-DR4 or DR7) but lacked HLA-DQ2 were at increased risk for ABPA after exposure to A. fumigatus. Within HLA-DR2 and HLA-DR5, restricted genotypes, in particular, HLA-DR2 B1*1501 and 1503 and HLA-DR5 B1*1101 and B1*1103, provide high relative risk. On the other hand, 40% to 44% of non-ABPA atopic Aspergillus-sensitive individuals have the HLA-DR2 and/or DR5 type. These results demonstrating HLA-DR restriction are similar to those found with purified house dust mite allergens [20–22]. Thus, certain genotypes of HLA-DR2 and DR5 may be necessary but not sufficient to cause ABPA. Furthermore, Chauhan et al  demonstrated that Asp f l allergen has a low-affinity of binding to HLA-DR. This is consistent with Th2 T cell response previously reported by others in that strong antigen HLA-DR-Ag-TCR affinity binding favored a Th1 cellular response whereas low affinity binding favored a Th2 humoral response [21–25]. Thus, we propose that antigen presenting cells bearing HLA-DR2/DR5 and with increased sensitivity to IL-4 stimulation may play a critical role in skewing the Th2 responses to A. fumigatus in ABPA.
We previously reported that in ABPA patients there was significant up-regulation of CD23 expression of IL-4 stimulated peripheral blood mononuclear cells . Other investigators have identified IL-4Rα single nucleotide polymorphisms (SNP) that result in a gain-of-function of IL-4 stimulation [8–16]. We examined one extracellular and four cytoplasmic SNPs. In these studies, we observed IL-4Rα SNPs in 95% of ABPA patients. The extracellular IL-4 binding SNP, ile75val, was the most frequently identified SNP and was frequently homozygous. Hershey's group reported that the combination of two variants, ile75val and gln576arg, together resulted in elevated IL-4 dependent CD23 expression and risk of atopy and asthma severity which was not observed when these SNPs were present alone . However, we observed the combination of ile75val and a cytoplasmic IL-4Rα SNP in only 28% of ABPA patients.
Other genetic risk factors have been proposed for the development of atopy, asthma and ABPA. In particular, in the development of increased asthma severity, investigators have reported that another combination of the ile75val IL-4Rα SNP and arg110gln IL-13 SNP has also been associated with allergy and asthma [26, 27]. We have recently begum studies to examine the IL-13 SNP. Saxena et al  reported that ABPA patients with polymorphisms (ala91pro, arg94arg) in the collagen region of pulmonary surfactant protein A2 (SP-A2) had more elevated total IgE levels and higher percentages of eosinophilia than those observed in patients who lacked the SNPs. They also found that 80% of patients carrying both alleles had ABPA (P = 0.0079, OR = 10.4), while only 50% and 60% of patients carrying each allele, individually, were ABPA subjects, suggesting an additive effect.
IL-10 may also play a significant role in the development of atopy and ABPA. In previous studies, we observed increased IL-10 and IL-5 synthesis of Asp f2, f3 and f4 stimulated peripheral blood lymphocytes in CF and asthmatic ABPA patients . Polymorphisms of the IL-10 promoter have been associated with risk of asthma  and ABPA . In particular, Brourad et al  reported the association of the -1082GG genotype of IL-10 promoter and the colonization with A. fumigatus and the development of ABPA in CF. The -10822G polymorphism has been associated with increased IL-10 synthesis; whereas the -1082A allele has lower IL-10 synthesis. These investigators reported that 23–25% of CF patients were -1082GG, 49–53% -1082AG, and 24–26% -1082AA. There was significantly increased risk to become colonized with A. fumigatus and to develop ABPA in the -1082GG genotype group, which developed in 7% of the patients. Thus, one may hypothesize that antigen presenting cells (dendritic cells) expressing HLA-DR2/DR5 with increased IL-10 synthesis and increased sensitivity to IL-4 stimulation due to IL-4Rα polymorphisms may be responsible for driving an allergic inflammatory response to A. fumigatus in ABPA.
The major limitation of this study is the small numbers of ABPA patients studied. However, the frequency of IL-4Rα polymorphisms is so prevalent, further studies will likely confirm this preliminary study.