Immunology & Immunological Disorders Poster Session



Materials & Methods


Discussion & Conclusion



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Bactericidal Activity of Human Milk Complement System

Contact Person: Michael O. Ogundele (mogundel@yahoo.com)

Discussion and Conclusion

The foregoing results highlight a significant contribution of the C system to the observed bacteriolytic activities of HBM. Heat-inactivation at 56C for 30 min as well as addition of inhibitory levels of EDTA both led to significant loss of HBM bactericidal activities. The findings also support the possible contribution of lysozyme, acting in synergism with sIgA antibodies and C in effecting bacteriolysis of E. coli bacteria (20).

Immunochemical and haemolytic levels of C system in the HBM have been fairly well studied by several authors. Using radial immunodiffusion assays, levels of C3 and C4 comparable to those in the normal serum have been found in the early colostrum. While C4 level fell precipitously from about 0.18mg/ml to 0.01mg/ml during the first month of lactation to relatively stable low levels of C3 and C4 in mature milk up till 18 months of lactation (10,24). The level of breast-milk C3 averages 6-7% that of the normal human serum while C4 averages 9-12% in most reported studies. C1 and C5 could not be detected immunochemically in the colostrum by Ballow et al. who conducted one of the earliest studies on breast-milk complement (9). By using high degree of concentration, C1q was later detected in the human colostrum (23). Up to a ten-fold difference between the immuno-chemical levels of C3 and C4 that has been reported by various authors, probably reflecting an inherent variability of the rate of transport and or production of these proteins in the breast-milk among different populations (5,10). It might however also reflect differences arising from different assay methods.

The level of factor B that has been measured immuno-chemically averages 2.5% that of the normal human serum (11). Evidence for an intact APC activity in the breast-milk however suggests that factor D is also present in physiologically significant levels (8). An approximately level of 15-20% C3 pro-activator (factor B) functional activity was detected in the colostrum when compared with that of the normal human serum (9).

The classical studies by Nakajima and other showed the presence of haemolytic activity of all the nine components of the classical complement components in the colostrum, ranging from 0.03 to 7% that of the normal human serum. Evidences for the presence of the APC factors were also obtained (8).

Only few studies have conclusively investigated the physiological functions of the HBM complement (2,15). The complement content of the HBM was shown to induce a respiratory burst (as evidenced by increased chemo-illuminiscence) in PMN leucocytes phagocytizing adherent bacteria, but not primarily responsible for its opsonizing properties (15). It has generally been argued that the complement activity in the breast-milk was insignificant because of the relatively low levels of most of the individual components and because of many other "anti-complement" factors present in the breast-milk (26-29). Moreover, the application of standard methods of assessing the serum complement are unreliable in the studies of breast-milk complement. These and many other peculiar problems associated with handling HBM samples have hampered further research into the possible contribution of the milk complement to the observed protection of the nursing infant against many infectious diseases (38).

It has recently been shown in cows milk that the complement factors in the milk are capable of being activated by the alternative pathway even in the absence of any inflammation, making use of a mastitis causing bacteria as a template (30). Similar findings have been reproduced with HBM in the present study. Though the contribution of C to the bactericidal activities of human breast-milk (HBM) has not been previously directly studied, evidences could be deduced.

The bacteriostatic effect of fresh breast-milk on bacterial growth at 37C was found to be abolished by heat treatment (31). Bacteriostatic activity of the frozen milk also deteriorated slightly after 1 month of storage at -20C (32). The observation that the bacterial count in the breast-milk decreased during in vitro storage at 4C (32) suggests that an anti-microbial process is activated during this period. In addition to the lipolysis-induced cytolytic effect, other complement-dependent mechanisms could also be responsible. Specific IgG directed against virulence factors of bacteria are present in the HBM (4,33). It is conceivable that exposure to these bacteria in the gastro-intestinal tract of the infant could lead to the activation of the classical pathway of C and could assist in the elimination of these organisms and preventing colonisation or infection.

Since similar bactericidal FFA are also released after the digestion of cows milk-based formula feed, the recognised differences between the occurrences of gastrointestinal infections between the breastfed and artificial fed infants tend to throw doubt on the possible physiological significance of the anti-microbial effect of these lipids (16). The physiological significance of breast-milk complement system, in relation to its presently observed bactericidal activity, might account for the generally observed trend of reduced total faecal bacterial load in the breastfed infant, particularly within the first week of life (34).

A deficient production of local complement components has been associated with an increased risk of developing mastitis in lactating mothers (35). Skin sepsis in the suckling infant, as well as inflammation of the lactating breast (mastitis), are associated with an increased level of complement secretion in the breast-milk (36). Though these are no proofs of cause-effect relationships, they point to the possible physiological role of the complement system on the mucosa surfaces.

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Ogundele, M.O.; (1998). Bactericidal Activity of Human Milk Complement System. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Available at URL http://www.mcmaster.ca/inabis98/immunology/ogundele0172/index.html
© 1998 Author(s) Hold Copyright