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

It could be argued that the use of the homologous Mhf isolate for experimental challenge of the recipient cats that had been used to infect the plasma donor cats contributed to the more pronounced immune response in the passively immunized cats. This has been reported for other feline pathogens, i.e. feline coronavirus (FCoV) which can cause feline infectious peritonitis (FIP) [44] . During the pathogenesis of FIP, the virus targets macrophages and infection of these cells can be enhanced in the presence of antibodies (antibody-dependent enhancement, ADE) [45] . Takano et al. [44] showed that cats passively immunized with antibodies to serotype I FCoV showed an enhanced onset of disease following inoculation with the homologous serotype; this was not found when cats passively immunized with antibodies to serotype II FCoV were challenged with serotype I. The authors concluded that FCoV re-infection with the same serotype might induce ADE and could advance the development of FIP. Although ADE has also been suspected in bacterial infection [46] , it is unknown whether it might play a role in the pathogenesis of feline hemoplasma infections [15] . Signs of infection enhancement have recently been documented in "Candidatus M. turicensis"-recovered, seropositive cats following a challenge with Mhf [15] . The study suggested that the presence of antibodies directed against CMt could enhance Mhf infection. However, to the best of our knowledge, disease enhancement after rechallenge with the same feline hemoplasma species has not been documented. In contrast, two studies showed that cats that had recovered from Mhf or CMt infection were protected from re-infection following re-challenge with the same hemoplasma species, respectively [27, 28] . Both studies had used an aliquot of the same Mhf or CMt isolate for re-infection that had been used for primary infection of the cats (personal communication, RHL).