Influence of genetic variation in host epithelial and immune cells on Chlamydia trachomatis infection and potential TFI induction. (A) Specific HLA*DQ alleles are correlated to TFI induction. (B) TFI patients carry an IL-10 promoter polymorphism. Cells from these individuals tend to produce less IL-10, more IFNγ and more TNF-α when leukocytes are stimulated in vitro. (C) IFN-γ induces IDO, which degrades tryptophan, an essential amino acid for both Chlamydia and the host cell. This may lead to dampening of the immune system, from a productive to a nonproductive chlamydial growth phenotype, which has been suggested to be associated with chronic infection and increased disease severity. IDO: Indoleamine 2,3-dioxygenase; TFI: Tubal factor infertility.