Lymphocyte Subpopulations in Breast-Fed and Formula-Fed Infants at Six Months of Age

Abstract

Factors such as cytokines and lymphocytes present in human milk can influence the developing immune system. This suggests an immunoregulatory role for human milk that is absent in infants consuming formula. There are very few data available from well-defined groups of breast-fed and formula-fed infants with regard to their immune status as reflected by lymphocyte immunophenotypic values. The aim of this study was to investigate the potential difference in lymphocyte subsets between breast-fed and formula-fed infants at 6 months of age. Blood samples were taken by venipuncture. Lymphocytes were analyzed by 2-color direct immunofluorescence with Becton Dickinson Immunocytometry Systems Simu1SET reagents (BD, Franklin Lakes NJ). There were 73 breast-fed infants and 55 formula-fed infants at 6 months of age. The frequency of natural killer (NK) cells (CD3^-/CD 16+ + CD56+) was greater in breast-fed infants (9.2%) than in formula-fed infants (6.6%, P < 0.001), while the CD4 to CD8 ratio was 2.8 in breast-fed infants compared with 3.4 in formula-fed infants (P < 0.001). Conclusion: Breast-fed infants (<250mL formula/bovine milk per week) had a greater proportion of NK cells and a lower CD4 to CD8 ratio than formula-fed infants at 6 months of age.

There are many compounds in human milk that may contribute to the protective effect of milk when consumed by infants, and it is tempting to speculate on the many mechanisms and interactions that must occur. Known immunological factors in human milk can be broadly divided into direct-acting antimicrobial factors, anti-inflammatory factors, and other bioactive compounds that are immunomodulating (Goldman 1993). The relatively recent identification of cytokines in human milk provides evidence of a family of immunomodulatory factors with the potential to influence the growth and development of the infant’s immune system. Interleukin 1 beta (IL-1f3) and tumor necrosis factor alpha (TNF-a) are among the number of peptide mediators identified in milk, and both often act synergistically to induce local inflammatory changes and mediate systemic acute-phase responses to microbial invasion and tissue injury (Munoz et al. 1990; Rudloff et al. 1992). Interleukin 6 (IL-6) plays a critical role in the growth and differentiation of B cells and has been associated with production of IgA in human milk (Saito et al. 1991). Growth factors have also been identified such as transforming growth factor beta (TGF-ß), which induces oral tolerance, stimulates IgA production, and has an important role in the maintenance of the integrity of the intestinal epithelial barrier (van Vlasselaer et al. 1992; Planchon et al. 1994). These and other cytokines are amongst the many bioactive substances in human milk that may contribute to the differences in lymphocyte subpopulations between breast-fed infants and formula-fed infants described in this paper.

In the first year of life a reduction in the incidence of illness in breast-fed infants compared with formula-fed infants has been demonstrated, even in affluent developed countries. Human milk clearly is a mixture of many factors that may have an important role in the protection of the infant against infection and in the development of the infant’s own immune system. The immunophenotypic differences between infants whose primary source of nutrition was human milk and those who were breast-fed for under 4 weeks are consistent with reported age-related changes, suggesting a greater “maturity” in the development of the immune system of breast-fed infants. These data provide additional evidence for the importance of maintaining breast-feeding rates in our society.