Cellular Interactions and the Secondary Response in vitro

Abstract

ALTHOUGH the role of cellular cooperation in the induction of the immune response has become firmly established only recently¹, morphological evidence suggesting that such cooperation takes place is quite old. Reports² of the aggregation of lymphoid cells around macrophages³ have been confirmed: “islets”, “rosettes” or “clusters” in cultures of cells (derived from humans⁴, guinea-pigs⁵, rabbits⁶ or mice⁷) stimulated with antigen or PHA⁸ were reported. We have investigated cluster formation to ascertain its relationship, if any, to the antigen-induced stimulation of sensitized cells⁹. We used peripheral blood leucocytes from rabbits immunized to bovine serum albumin (BSA) or to human red blood cells (HRBC). The BSA was given in complete Freund's adjuvant (three intramuscular injections of 7.5 mg BSA each, into the hind legs at weekly intervals). HRBC (1 ml.) was given once into the ear vein, as a 20% suspension in saline. Cell cultures and ³H-thymidine incorporation were measured as before¹⁰. To prepare cell smears, cells were washed three times and suspended in one drop of normal rabbit serum and 1 µl. of the suspension was spread on a microscope slide. This ensured a reasonably constant number of cells per slide and made possible comparisons between different experiments. Smears were fixed with methanol and stained with Giemsa.