Abstract The interaction between the enhancing and neutralizing activities of three monoclonal antibodies (MAbs) (5‐6‐2, 6‐4‐2 and 7‐4‐1) to the spike protein of feline infectious peritonitis virus (FIPV) strain 79‐1146 was determined using feline macrophages. At a high MAb concentration, all of the three MAbs completely inhibited the FIPV infection at 37 C. However, two of them (6‐4‐2 and 7‐4‐1) enhanced FIPV infection when either the MAb concentration or reaction temperature was lowered. These MAbs also exerted an immediate infectivity‐enhancing activity for up to 10 min of reaction and by 20 min, neutralizing activities were observed. Only MAb 5‐6‐2 consistently showed neutralizing activity regardless of the reaction conditions. Competition with sera from cats experimentally infected with FIPV strain 79‐1146 or feline enteric coronavirus strain 79‐1683 showed that the two epitopes recognized by MAb 5‐6‐2 and MAb 6‐4‐2, respectively, are also recognized by the natural host.
Unmethylated CpG-ODN are known to enhance Th1-type immune response. However, optimal sequences of CpG-ODN for activating Th1-type immune cells vary among species. It is necessary to identify the effective CpG-ODN sequences in each species. In the present study, in order to identify the sequences of CpG-ODN that produce fIFN-γ in cats, 14 kinds of ODN were synthesized and examined regarding their ability to induce fIFN-γ in feline PBMC and splenocytes. It was shown that some CpG-ODN significantly induced fIFN-γ production in splenocytes, but not in PBMC. We found that three kinds of CpG-ODN (no. 2, 5'-ggTGCATCGATGCAGggggG-3'; no. 5, 5'-ggTGCGTCGACGCAGggggG-3'; no. 10, 5'-ggTGCTACGTAGCAGggggG-3') specifically and significantly induced fIFN-γ production in feline splenocytes. The reverse sequences, GpC-ODN, do not cause significant fIFN-γ production. The fIFN-γ production inductivity of a mixture of CpG-ODN nos. 2, 5 and 10 was higher than those of individual CpG-ODN. When the CpG-ODN mixture was encapsulated in an MCL and administrated to cats, the number of fIFN-γ(+) cells in PBMC significantly increased. CpG-ODN nos. 2, 5 and 10 should be useful to elicit a Th1-type immune response as a vaccine adjuvant in cats.
