Avian metapneumovirus subtype C (aMPV/C) causes a severe upper respiratory tract (URT) disease in turkeys. The disease is characterized by viral replication and extensive lymphoid cell infiltrations in the URT. The identity of infiltrating cells and their possible involvement in the immunopathogenesis of the disease are not known. The role of local mucosal immunity in viral defense has not been examined for aMPV/C. The overall objective of the study was to examine the immunopathogenesis of aMPV/C in ovo and hatched turkeys, with emphasis on the involvement of local mucosal immunity in viral defense. Three specific objectives were pursued. First, the immune cells, especially mucosal T cells that infiltrate the URT of turkeys following aMPV/C exposure were characterized. Two-week-old aMPV/C antibody-free turkeys were inoculated oculonasally (O/N) with live aMPV/C. At 5 and 7 days post inoculation (DPI), lymphoid cells infiltrating the mucosal lining of the turbinates of the virus-exposed and untreated control turkeys were isolated by enzymatic treatment. In the URT, aMPV/C exposure increased the proportion of CD8+ T cells but not of CD4+ T cells. In addition, CD8 gene expression was upregulated after virus exposure whereas CD4 gene expression remained unchanged. At 5 and 7 DPI, aMPV/C-exposed turkeys showed upregulated gene expression of IFN-gamma and IL-10 in the turbinate tissue. These results suggested that aMPV/C modulated local cellular immunity in the URT of turkeys. Secondly, the ability of an adjuvanted inactivated aMPV/C (Ad-iaMPV/C) inoculated by the respiratory route to induce protective mucosal immunity in the URT was examined. aMPV/C antibody-free turkeys were inoculated via the O/N route with inactivated virus adjuvanted with synthetic double-stranded RNA polyriboinosinic polyribocytidylic acid (Poly IC). Ad-iaMPV/C immunized turkeys showed an increased number of mucosal IgA+ cells in the URT and increased levels of virus-specific IgG and IgA in the lachrymal fluid and serum. After 7 or 21 days post immunization, turkeys were challenged with pathogenic aMPV/C via the O/N route. Turkeys immunized with Ad-iaMPV/C were protected against microscopic lesions and the replication of the challenge virus in the URT. These observations revealed that inactivated aMPV/C administered by the respiratory route induced protective immunity against challenge with the pathogenic virus. As the last objective, we studied the immunopathogensis and protective immunity of aMPV/C in turkeys following in ovo exposure. aMPV/C was inoculated into commercial aMPV/C antibody-free turkey eggs via the amniotic route at embyronation day (ED) 24. Hatchability of eggs was not affected by the virus inoculation. At the day of hatch (ED 28) (4DPI) and 5 days post hatch (9DPI), the virus genome was detected by qRT-PCR in the turbinate, trachea and lung but not in the thymus or the spleen. Turbinate mucosa had mild lymphoid cell infiltration, and there were no detectable lesions in the lung. Spleen cells and thymus cells from virus-exposed turkeys responded poorly to T cell mitogens. In addition, IFN-gamma and IL-10 gene expression was increased in the turbinate tissue of virus-exposed turkeys. In ovo virus exposure increased the levels of aMPV/C-specific IgG in the serum and the lachrymal fluid. At 3 weeks of age, the in ovo immunized turkeys were protected against a challenge with pathogenic aMPV/C. These data indicated that in ovo vaccination may be used in turkeys to control aMPV/C.