Protective Immunity against Infection with Mycoplasma haemofelis

Hemoplasmas are potentially zoonotic mycoplasmal pathogens, which are not consistently cleared by antibiotic therapy.Myco-plasma haemofelis is the most pathogenic feline hemoplasma species. The aim of this study was to determine how cats previously infected withM. haemofelis that had recovered reacted when rechallenged withM. haemofelis and to characterize the immune response following de novo M. haemofelis infection and rechallenge. Five specific-pathogen-free (SPF)-derived naive cats (group A) and five cats that had recovered fromM. haemofelis infection (group B) were inoculated subcutaneously withM. haemofelis. BloodM. haemofelis loads were measured by quantitative PCR (qPCR), antibody response to heat shock protein 70 (DnaK) by enzyme-linked immunosorbent assay (ELISA), blood lymphocyte cell subtypes by flow cytometry, and cytokine mRNA levels by quantitative reverse transcriptase PCR. Group A cats all became infected with high bacterial loads and seroconverted, while group B cats were protected from reinfection, thus providing the unique opportunity to study the immunological parameters associated with this protective immune response againstM. haemofelis. First, a strong humoral response to DnaK was only ob-served in group A, demonstrating that an antibody response to DnaK is not important for protective immunity. Second, proin-flammatory cytokine interleukin-6 (IL-6) mRNA levels appeared to increase rapidly postinoculation in group B, indicating a pos-sible role in protective immunity. Third, an increase in IL-12p35 and-p40 mRNA and decrease in the Th2/Th1 ratio observed in group A suggest that a Th1-type response is important in primary infection. This is the first study to demonstrate protective im-munity againstM. haemofelis reinfection, and it provides important information for potential future hemoplasma vaccin