Adverse immunological imprinting by cytomegalovirus sensitizing for allergic airway disease

Cytomegalovirus (CMV) infection has a profound impact on the host's immune system. Immunological imprinting by CMV is not restricted to immunity against CMV itself, but can affect immunity against other viral or non-viral infectious agents and also immunopathological responses. One category is heterologous immunity based on molecular mimicry, where antigen recognition receptors specific for a CMV antigen with broad avidity distribution also bind with some avidity to unrelated antigens and exert effector functions against target structures other than those linked to CMV. Another category is induction of cytokines by CMV infection that inhibit or drive immune responses to bystander antigens unrelated to CMV, and a third category is the activation of antigen-presenting cells by CMV from which unrelated antigens profit as "stowaways". A striking example of the "stowaway" category, actually one that is of medical importance, has been published recently and will be discussed here for the more general reader. Specifically, in a murine model, CMV airway infection and inhaled environmental antigen of poor intrinsic allergenic potential were found to sensitize for allergic airway disease (AAD) only when combined. As to the mechanism, viral activation of CD11b⁺ conventional dendritic cells (CD11b⁺ cDC) that localize to airway mucosa facilitates uptake and processing of inhaled antigen. Thus, CMV serves as a "door opener" for otherwise harmless environmental antigens that have no intrinsic property to activate DC. Antigen-laden CD11b⁺ cDC migrate selectively to the airway draining lymph nodes, where they prime type-2 CD4⁺ T helper (Th-2) cells. Upon airway re-exposure to the inhaled antigen, Th-2 cells secrete interleukins (IL-4, IL-5, IL-9, and IL-25) known to induce goblet cell metaplasia, the lead histopathological manifestation of AAD that is characterized by thickening of airway epithelia and increased numbers of mucus-producing goblet cells, resulting in enhanced mucus secretion and airflow obstruction.