Enhancement of Streptococcus pneumoniae serotype 6B infection in mice after passive immunization with human serum.

Passive immunization in an experimental pneumococcal infection model has been proposed as a way to further characterize the protective capacity of human post vaccination sera. However, in experiments described in the present paper, we found that the same human immune serum that induced some degree of protection when NIHS mice were challenged with one strain of Streptococcus pneumoniae serotype 6B, did not confer protection when the mice were challenged with another strain of serotype 6B. On the contrary, with this bacterial strain, six different human sera appeared to enhance pneumococcal infection leading to higher levels of bacteremia and more rapid death of passively immunized than of non-immunized mice. In contrast, mice passively immunized with mouse immune serum showed reduced bacteremia and enhanced survival after challenge with the same dose of the same strain of pneumococci. The enhancing property of human serum did not seem to be caused by anti-type 6B antibodies. However, we cannot exclude the possibility that human antibodies are less protective than mouse antibodies against type 6B infection. Our results indicate that negative results in passive immunization experiments with human sera should be interpreted with caution.