Generation of complement fragment C5a in milk is variable among cows

The appearance of chemotactic fragments of complement at sites of infection is an important component of innate immunity. The contribution of C5a, the most biologically active complement fragment, to the recruitment of phagoc ytes in milk is not well defined, in particular the amount of C5a that is r eleased in normal milk before inflammation. The generation of C5a in normal milk upon activation of complement by invading bacteria depends on the amo unt of available C5 and on the activity of the C3/C5-convertase of the alte rnative pathway. Concentrations of C5 were measured in one fore and one rea r uninfected quarter of 19 Holstein cows. Values were consistent within cow s, but widely dispersed among cows (0.19 to 1.94% blood concentration). C5 concentrations in milk were loosely related to concentrations in blood. By comparison, the range of milk concentrations of C3 (1.4 to 4.4%, mean 2.46 +/- 0.63% of blood concentration) was narrower. Two groups of six cows with high milk concentrations of C5 (cows H5: mean = 1.31%) and six cows with l ow milk concentrations of C5 (cow L5: mean = 0.21%) were constituted for fu rther analysis of complement activation. There was a positive correlation b etween concentrations in milk of BSA and C5, but not between concentrations of BSA and C3. The activities of the C3- and C5-convertases were assessed through the deposition on complement-activating bacteria (Streptococcus aga lactiae) of C3 and C5 fragments, respectively. The deposition of C3 was 1.7 -fold higher, and the deposition of C5 was 2.75-fold higher in milk from H5 cows than in milk of L5 cows. Higher concentrations of C5 and better funct ioning of C5-convertase were mirrored by a much higher concentration of C5a in milk from H5 cows (12.30 ng/ml) than in milk of L5 cows (0.76 ng/ml) af ter activation of complement with zymosan. These results indicate that cows differed widely in their capacity to generate C5a in milk before inflammat ion, and that milk C5 concentrations were a primary limiting factor for C5a generation. Cows with the lowest milk concentrations of C5 are likely unab le to use the complement system for the initial recruitment of leukocytes.