Study of the Humoral Immune Response Induced by a Polyvalent Vaccine in Cattle Naturally Exposed to Leptospira spp. Infection

Abstract

This study examines Holstein heifers' immune response to a commercial vaccine formulated with eight Leptospira spp. antigens. Selected from a farm with 4.5% leptospiruria prevalencein adult cows, sixty heifers were tested negative in two consecutive MATs for 24 serovars (cutoff=50) and urine qPCR analyses. Thirty heifers received the vaccine (Vaccinated Group, VG), while the remaining did not (Control Group, CG). The initial dose was administered at 3-8 months of age followed by three boosters at months 1, 7 and 15 post-first dose. Blood and urine samples were collected monthly over 15 months. Leptospiruria was detected through qPCR and culture (EMJH-medium). MAT measured antibodies against vaccine antigen serovars (homologous), and others circulating in Uruguayan cattle (heterologous). IgG levels were also assessed by ELISA. Antibody neutralization was evaluated using both in vitro assays and in vivo murine infection models. Leptospiruria was observed in both groups, with no significant difference between VG and CG (p>0.05). The vaccine showed 20% efficacy in inhibiting Leptospira excretion. MAT reactivity in VG sera indicated immunogenicity for seven out of eighthomologous antigens. Heterologous antigens showed low MAT seroreactivity, except for L. interrogans serogroup Pomona variant Kennewicki, although it was significantly lower than the vaccine homolog L. interrogans serogroup Pomona serovar Pomona (p<0.0001). IgG levels increased post-boost, indicating a memory humoral response. Vaccine-induced antibodies exhibited in vitro neutralization activity against autochthonous variants: L. interrogans serogroup Canicola serovar Canicola and L. borgpetersenii serogroup Sejroe serovar Hardjo. However, when passively transferred into the murine infection model, antibodies failed to prevent renal colonization by L. interrogans serogroup Canicola serovar Canicola and L. interrogans serogroup Pomona serovar Kennewicki. In conclusion, while the vaccine elicited a humoral immune response, the efficacy in preventing infection and renal colonization was limited.