Intramuscular but not nebulized administration of a mRNA vaccine against Rhodococcus equi stimulated humoral immune responses in neonatal foals

OBJECTIVE Design and evaluate immune responses of neonatal foals to a mRNA vaccine expressing the virulence-associated protein A (VapA) of Rhodococcus equi. ANIMALS Cultured primary equine respiratory tract cells; Serum, bronchoalveolar lavage fluid (BALF), and peripheral blood mononuclear cells (PBMCs) from 30 healthy Quarter Horse foals. METHODS VapA expression was evaluated by western immunoblot in cultured equine bronchial cells transfected with 4 mRNA constructs encoding VapA. The mRNA construct with greatest expression was used to immunize foals at ages 2 and 21 days in 5 groups: (1) 300 μg nebulized mRNA (n = 6); (2) 600 μg nebulized mRNA (n = 4); (3) 300 μg mRNA administered intramuscularly (IM) (n = 5); (4) 300 μg VapA IM (positive controls; n = 6); or (5) nebulized water (negative controls; n = 6). Serum, BALF, and PBMCs were collected at ages 3, 22, and 35 days and tested for relative anti-VapA IgG₁, IgG_4/7, and IgA activities using ELISA and cell-mediated immunity by ELISpot. RESULTS As formulated, nebulized mRNA was not immunogenic. However, a significant increase in anti-VapA IgG_4/7 activity (P <.05) was noted exclusively in foals immunized IM with VapA mRNA by age 35 days. The proportion of foals with anti-VapA IgG₁ activity > 30% of positive control differed significantly (P =.0441) between negative controls (50%; 3/6), IM mRNA foals (100%; 5/5), and IM VapA (100%; 6/6) groups. Natural exposure to virulent R equi was immunogenic in some negative control foals. CLINICAL RELEVANCE Further evaluation of the immunogenicity and efficacy of IM mRNA encoding VapA in foals is warranted.