Coincident airway exposure to low-potency allergen and cytomegalovirus sensitizes for allergic airway disease by viral activation of migratory dendritic cells

Despite a broad cell-type tropism, cytomegalovirus (CMV) is an evidentially pulmonary pathogen. Predilection for the lungs is of medical relevance in immunocompromised recipients of hematopoietic cell transplantation, in whom interstitial CMV pneumonia is a frequent and, if left untreated, fatal clinical manifestation of human CMV infection. A conceivable contribution of CMV to airway diseases of other etiology is an issue that so far attracted little medical attention. As the route of primary CMV infection upon host-to-host transmission in early childhood involves airway mucosa, coincidence of CMV airway infection and exposure to airborne environmental antigens is almost unavoidable. For investigating possible consequences of such a coincidence, we established a mouse model of airway co-exposure to CMV and ovalbumin (OVA) representing a protein antigen of an inherently low allergenic potential. Accordingly, intratracheal OVA exposure alone failed to sensitize for allergic airway disease (AAD) upon OVA aerosol challenge. In contrast, airway infection at the time of OVA sensitization predisposed for AAD that was characterized by airway inflammation, IgE secretion, thickening of airway epithelia, and goblet cell hyperplasia. This AAD histopathology was associated with a T helper type 2 (Th2) transcription profile in the lungs, including IL-4, IL-5, IL-9, and IL-25, known inducers of Th2-driven AAD. These symptoms were all prevented by a pre-challenge depletion of CD4(+) T cells, but not of CD8(+) T cells. As to the underlying mechanism, murine CMV activated migratory CD11b(+) as well as CD103(+) conventional dendritic cells (cDCs), which have been associated with Th2 cytokine-driven AAD and with antigen cross-presentation, respectively. This resulted in an enhanced OVA uptake and recruitment of the OVA-laden cDCs selectively to the draining tracheal lymph nodes for antigen presentation. We thus propose that CMV, through activation of migratory cDCs in the airway mucosa, can enhance the allergenic potential of otherwise poorly allergenic environmental protein antigens. Author summary From an epidemiological perspective, natural host-to-host transmission of human CMV mostly occurs in early childhood through saliva from virus-shedding intimate contact persons, such as family members or peers. The oronasal route of transmission involves also the airway mucosa and airway-draining lymph nodes. Almost unavoidably, CMV airway infection coincides with airway exposure to environmental antigens, which include potent classical allergens but also protein antigens that have low-to-no allergenic potential on their own. Ovalbumin (OVA), when administered as a purified protein, can serve as a well-studied model antigen for only poorly allergenic environmental antigens. In a murine model of airway exposure to OVA for allergenic sensitization, we have addressed the question if a simultaneous airway infection with murine CMV (mCMV) can promote allergic airway disease (AAD) elicited by challenge exposure to OVA. As anticipated by the model design, exposure to OVA alone did not sensitize for an allergic response to challenge exposure, nor did mCMV infection alone. Notably, based on viral activation of antigen uptake by DCs, both combined sensitized for a type 2 CD4(+) T helper (Th2) cell-driven AAD histopathology. This is a novel aspect in CMV pathobiology with the medical relevance that CMV airway infection enlarges the spectrum of environmental allergens.