A variety of leucocytes occur in colostrum (~ 4 x 106 ml-1) and later in milk (~ 105 ml-1). Macrophages (55-60%) and polymorphonuclear granulocytes (30-40%) dominate over lymphocytes (5-10%), the latter being mainly (75-80%) T lymphocytes (Goldman, 1993; Wold and Hanson, 1994). Oral administration of macrophages in newborn mice showed survival of these cells for several hours in the gut and even some mucosal uptake (Hughes et al., 1988). Experiments with milk leucocytes in newborn rats, calves and lambs likewise demonstrated transepithelial migration, lymphocytes apparently being the predominant cell type (Slade and Schwartz, 1987). Also, the distribution of labelled human colostral leucocytes after enteral administration in premature baboons suggested epithelial adherence in the gut, as well as mucosal uptake and persistence for more than 60 h, along with some peripheral migration (Jain et al., 1989). The contribution of milk lymphocytes to the infant's developing intestinal immune system therefore remains an intriguing possibility.
Milk leucocytes generally express markers of previous priming and respond readily to restimulation. The macrophages contain engulfed SIgA that they may release on contact with bacteria in the gut (Slade and Schwartz, 1987). They may also secrete an array of immunologically important cytokines (Wold and Hanson, 1994). It is furthermore of interest that, compared with peripheral blood, breast milk is relatively enriched in T lymphocytes with the alternative TCR7/8; this subset mainly employs the V81/J81-encoded receptor, as do intraepithelial 7/8 T-cells in the gut (Bertotto et al., 1991). T lymphocytes of this phenotype with specificity for Mycobacterium tuberculosis appear in colostrum of tuberculin-positive nursing mothers (Bertotto et al., 1993). This observation is interesting in view of the direct antimicrobial activity mediated by various T-cell phenotypes upon interaction with microbial targets (Levitz et al., 1995). In this respect, mucosal defence may especially engage 7/8 T-cells. Their supply from breast milk could thus be immunologically important before the baby's IEL population has developed numerically and functionally.
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