IFN induction and associated changes in splenic leukocyte distribution.

Our studies were undertaken to evaluate early events associated with IFN expression and IFN-induced cellular changes in the spleen. Polyinosinic-polycytidylic acid (poly(I:C)) was used to induce IFN in C57BL/6 mice. Biologically active IFN was present in spleens and sera of poly(I:C)-treated mice as early as 3 h and peaked at 6 to 12 h posttreatment. Neutralization assays and Northern blot analyses demonstrated that IFN-beta was the form of IFN preferentially induced in the spleens. Immunohistochemical staining and in situ hybridization studies of splenic tissue sections demonstrated that, although many cells were induced to express low levels of IFN-beta, low frequency cells were particularly potent expressers of IFN-beta. Perivascular cells expressed high levels of IFN-beta. Other positive cells were localized in red pulp at early times and in white pulp at later times posttreatment. Histologic examination of splenic sections showed that the IFN production was accompanied by dramatic architectural changes. There were significant 1.4-fold increases in the proportion of white pulp area and > 50% decreases in red pulp leukocyte number. These architectural changes appeared at 6 h and lasted through 36 h after poly(I:C) treatment. They were not due to increased cell proliferation as splenic weights and cell yields were not elevated. The changes in splenic leukocyte distribution were shown to be a result of IFN induction as: 1) treatment with anti-IFN antibodies inhibited poly(I:C)-induced depletion of red pulp leukocytes, and 2) administration of purified IFN-beta induced both an increase in white pulp area and a decrease in red pulp leukocytes. Splenic leukocytes were labeled with the lipophilic fluorescent dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate, to evaluate cell traffic after IFN induction. Poly(I:C) enhanced accumulation of cells in white pulp regions. In contrast to the 76 +/- 2 labeled cells that accumulated in 0.5 mm2 of white pulp area in control PBS-treated mice, 131 +/- 4 labeled cells accumulated in 0.5 mm2 of white pulp area in poly(I:C)-treated mice. The poly(I:C)-enhanced leukocyte accumulation in short term cell migration to white pulp regions was dependent on preexposure of both splenic leukocytes and recipient environments to the IFN inducer. These data demonstrate that poly(I:C) is a potent and rapid inducer of IFN-beta production by splenic cells and that IFN induces cellular redistribution in the spleen. The results suggest that margination of leukocytes into splenic white pulp may be another important immunoregulatory function of IFN.