ENHANCEMENT OF STREPTOCOCCUS-PNEUMONIAE SEROTYPE 6B INFECTION IN MICEAFTER PASSIVE-IMMUNIZATION WITH HUMAN SERUM

Passive immunization in an experimental pneumococcal infection model h as been proposed as a way to further characterize the protective capac ity of human post vaccination sera. However, in experiments described in the present paper, we found that the same human immune serum that i nduced some degree of protection when NIHS mice were challenged with o ne strain of Streptococcus pneumoniae serotype 6B, did not confer prot ection when the mice were challenged with another strain of serotype 6 B. On the contrary, with this bacterial strain, six different human se ra appeared to enhance pneumococcal infection leading to higher levels of bacteremia and more rapid death of passively immunized than. of no n-immunized mice. In contrast, mice passively immunized with mouse imm une serum showed reduced bacteremia and enhanced survival after challe nge with the same dose of the same strain of pneumococci. The enhancin g property of human serum did not seem to be caused by anti-type 6B an tibodies. However, we cannot exclude the possibility that human antibo dies are less protective than mouse antibodies against type 6B infecti on. Our results indicate that negative results in passive immunization experiments with human sera should be interpreted with caution. (C) 1 996 Academic Press Limited