The in situ immune response in popliteal lymph nodes of mice after macrophage depletion. Differential effects of macrophages on thymus-dependent and thymus-independent immune responses.

Mice were subcutaneously (SC) injected in the left hind footpad with dichloromethylene diphosphonate (Cl2MDP)-containing liposomes to eliminate macrophages lining the subcapsular sinus (SCS) and those in the medulla of draining popliteal lymph nodes (PLN). In order to study the effect of depletion of these macrophages on the in situ immune response in the PLN, liposome-treated mice were SC injected in the same footpad with thymus-independent (TI) type 1 antigen trinitrophenylated lipopolysaccharide (TNP-LPS), TI-type 2 antigen TNP-Ficoll or thymus-dependent (TD) antigen TNP-keyhole limpet haemocyanin (TNP-KLH). No major differences were observed in antibody-serum titers of liposome-treated and control animals. After primary as well as secondary immunization with the TD-antigen TNP-KLH, an increase in the number of antibody-forming cells (AFC) was found and the peak of response was delayed in the PLN of liposome-treated animals. Such differences were not observed with the TI-antigens. These results indicate that macrophages lining the SCS and those in the medulla of the PLN are not essential for the induction of an immune response. The positive effect of macrophage-depletion on the number of AFC may be explained by competition for the antigen by macrophages and other antigen-presenting cells.