Passive immunization does not provide protection against experimental infection with Mycoplasma haemofelis

Sugiarto, Sarah; Spiri, Andrea M.; Riond, Barbara; Novacco, Marilisa; Oestmann, Angelina; de Miranda, Luisa H. Monteiro; Meli, Marina L.; Boretti, Felicitas S.; Hofmann-Lehmann, Regina; Willi, Barbara

This is the first study to investigate the protective role of the humoral immune response against hemoplasma infection using a passive immunization experiment. Our study demonstrated that the passive transfer of antibodies from Mhf-recovered to naïve SPF cats does not prevent infection, high bacterial loads and the development of clinical signs following homologous challenge with Mhf. The passively immunized and control cats showed no differences in the onset and extent of bacteremia and anemia during the course of Mhf infection. Hicks et al. [28] recently documented that Mhf-recovered cats were protected from reinfection following re-challenge with a homologous Mhf isolate. Our study indicates that the presence of antibodies to Mhf cannot mediate protection against homologous challenge, but that cellular or innate immune mechanisms may be necessary to provide protection against Mhf infection. This is also in line with the study by Hicks et al. [28] , which reported protection from Mhf reinfection in the absence of a pronounced Th2-type response in the cats after re-challenge. In the latter study, the protected cats showed no increase in anti-DnaK antibody and IL-10 mRNA levels following Mhf re-challenge. The anti-DnaK antibody levels in the passively immunized cats were only higher in the first 2 weeks after transfusion than in the control cats, and they remained below the threshold for seropositivity up to day 28 pi. It is possible that the level of antibodies in the passively immunized cats following transfusion was not sufficiently high to prevent bacteremia. However, Hicks et al. [28] documented protection from Mhf although the antibody levels to DnaK did not rise after Mhf challenge infection and remained at rather low levels (mean log-transformed relative antibody level (RAL) < 1.2) when compared to the control cats with de novo Mhf infection (peak mean RAL > 2.2). They concluded that if Mhf-specific antibodies had been responsible for immune protection in these cats, then low levels of anti-DnaK antibodies could be sufficient to provide protection against Mhf infection. Because the assay and the calculations used to quantify anti-DnaK antibodies in that study were different from those used in the present study, a direct comparison of antibody titers was not possible. Furthermore, it needs to be considered that antibodies with different epitope specificities that are not detected with current DnaK ELISA could mediate immune protection. However, assays to measure such antibodies are not available.