T lymphocytes in colostrum and peripheral blood differ in their capacity to form thermostable E-rosettes

The cellular population of human breast milk collected within 5 days postpartum consists primarily of lipid laden polymorphonuclear leukocytes and macrophages, whereas lymphocytes comprise <10% of the total cells. A discontinuous density gradient technique was developed to obtain highly enriched (75 ± 6%) fractions of milk lymphocytes. The majority of milk lymphocytes are T cells as determined by their capacity to form rosettes at 4°C with sheep erythrocytes (E). The lymphocytic origin of the rosetted cells was confirmed by morphologic evaluation of Wright-Giemsa-stained cytocentrifuge slides. A novel finding is that a large proportion of milk T lymphocytes form E-rosettes at 37°C as well as at 4°C. The capacity to form thermostable E-rosettes is not a general property of T lymphocytes in the postpartum period, since peripheral blood T lymphocytes from milk donors form few thermostable E-rosettes. In addition, peripheral blood lymphocytes did not acquire the capacity to form thermostable E-rosettes after density gradient centrifugation nor after incubation with autologous milk. The finding that T lymphocytes in milk form thermostable E-rosettes suggests that milk T lymphocytes may be activated or immature. In either case, these data support the concept that lymphocytes that home to the mammary gland are a select subset of the total lymphocyte population.