Mannan-binding protein-like activity in the sera of newborn piglets

The mechanism of the antibody-independent bactericidal activity of the sera of newborn piglets deprived of colostrum was studied using rough strains of E. coli and S. typhimurium. Although all strains were invariably killed by all newborn piglet sera tested, two different mechanisms of killing were encountered. Using specific anti-pig C1q antiserum, strains S. typhimurium LT2Ml and E. coli K-12, strain Gal 23 were found to be killed by a C1q-dependent mechanism, while the killing of E. coli S 16, E. coli K-12, strain W 3100 and E. coli B 41 could not be inhibited by anti-C1q antiserum. In order to test whether a mannan-binding protein is responsible for the bactericidal effect in the latter group of strains, we examined the inhibitory activity of two types of mannans isolated from S. cerevisiae and H. capsulata, respectively. The use of a purified rabbit mannan-binding protein showed that only strains killed by the C1q-independent mechanism were sensitive to the MBP-dependent mechanism of killing, the inhibitory activities of both mannans were found to be equal. As expected, the inhibitory effect of mannan on the bactericidal activity of newborn piglet sera was also detected only in strains killed by the C1q-independent mechanism. Surprisingly, only the phosphomannan isolated from H. capsulata was found to be an effective inhibitor of the bactericidial activity of piglet sera against E. coli S 16 and E. coli K-12, strain W 3100, while the mannan isolated from S. cerevisiae was inactive. Hence the factor present in newborn piglet sera is either MBP with slightly different binding properties, or a completely different protein.