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

In the passively immunized cats, all analyzed lymphocyte subsets [CD5 + , CD4 + and CD8 + T lymphocytes, activated CD4 + T lymphocytes (CD4 + CD25 + ) and B lymphocytes (CD45R/B220 + )], showed a significant change over time (p F < 0.05). In the control cats, a significant change during the course of infection was only observed for the activated CD4 + T lymphocyte subset (p F < 0.05). Specifically, cats in group A showed a significant decrease in the CD4 + , CD8 + and CD5 + T lymphocyte counts, with the nadir reached at days 21, 21 and 28 pi, respectively, coinciding with the onset of peak bacteremia (CD4 + , p F = 0.0012; CD8 + , p F = 0.0002; CD5 + , p F < 0.0002, Figures 6A-C) . The CD4 + /CD8 + ratio was significantly increased at 2-3 weeks pi in the passively immunized cats (p F = 0.0002, Figure 6D ). Cats in group A showed a significant decrease in their B lymphocyte counts during maximal bacteremia, with the nadir reached at day 28 pi (p F = 0.0103, Figure 6E ), and the B lymphocyte counts were significantly higher at days 3, 14, 42 and 98 pi compared with cats in group B (p MWU < 0.05; Figure 6E ). When cat JHW3 was excluded from the analyses, the B lymphocyte counts were also higher in cats in group A compared to cats in group B, with significance reached at days 3 and 14 pi (P MWU < 0.05). The activated CD4 + T lymphocyte subsets showed a somewhat different pattern than the other lymphocyte subsets in group A. The activated CD4 + T lymphocyte counts increased early during the course of infection into the onset of PCRpositivity and peak bacteremia, and then, they markedly decreased and reached a nadir at day 42 pi (p F < 0.0001); this pattern was also present but less pronounced in group B (p F = 0.0249, Figure 6F ). Furthermore, compared with cats in group B, cats in group A showed significantly higher activated CD4 + T lymphocyte counts at day 14 pi (p MWU = 0.0400) and significantly lower levels at day 42 pi (p MWU = 0.0193; Figure 6F ). When cat JHW3 was excluded from the analyses, a tendency for higher activated CD4 + T lymphocyte counts in cats in group A compared to group B was found at day 14 pi (P MWU = 0.0539); the difference at day 42 pi was still significant (lower counts in cats in group A when compared to cats in group B; P MWU = 0.024).