Streptococcus pneumoniae and Listeria monocytogenes, pathogens which can cause severe
infectious disease in human, were used to infect
properdin-deficient and wildtype mice. The aim was to deduce a role for
properdin, positive regulator of the alternative pathway of complement activation, by comparing and contrasting the immune response of the two genotypes in vivo. We show that
properdin-deficient and wildtype mice mounted antipneumococcal serotype-specific
IgM antibodies, which were protective.
Properdin-deficient mice, however, had increased survival in the model of streptococcal
pneumonia and
sepsis. Low activity of the classical pathway of
complement and modulation of FcγR2b expression appear to be pathogenically involved. In
listeriosis, however,
properdin-deficient mice had reduced survival and a dendritic cell population that was impaired in maturation and activity. In vitro analyses of splenocytes and bone marrow-derived myeloid cells support the view that the opposing outcomes of
properdin-deficient and wildtype mice in these two
infection models is likely to be due to a skewing of macrophage activity to an M2 phenotype in the
properdin-deficient mice. The phenotypes observed thus appear to reflect the extent to which M2- or M1-polarised macrophages are involved in the immune responses to S. pneumoniae and L. monocytogenes. We conclude that
properdin controls the strength of immune responses by affecting humoral as well as cellular phenotypes during acute
bacterial infection and ensuing
inflammation.