PDI is also implicated in the attachment of some bacteria from different species of Chlamydia [72] [73] [74] . Chlamydia is an obligatory intracellular pathogen that causes diverse diseases in humans. The most common species are Chlamydia trachomatis, which is sexually transmitted and can cause blindness and infertility, and C. pneumoniae, which affects the respiratory tract. CHO cells have impaired endogenous PDI expression due to a defect in truncated mRNA processing, thus providing a valuable model to understand the effect of PDI-mediated cell-cell interaction and infection. These cells are very resistant to Chlamydia infection showing impaired attachment, an effect restored by ectopic expression of PDI [73] . Similar to HIV infection, the molecular mechanisms most likely include the reductive activity of PDI (reaction 1, Figure 1 ) on the surface of CHO cells [72] .