IFN-γ abrogates IL-7-dependent proliferation in pre-B cells, coinciding with onset of apoptosis

These studies investigated the mechanism by which interferon-γ (IFN-γ) inhibits the interleukin-7 (IL-7)-dependent proliferation of BALB/c bone marrow B-cell precursors in vitro. Low concentrations (1 U/ml) of recombinant murine IFN-γ (rmIFN-γ) caused a ~80% suppression of IL-7 colony-forming units (CFU) formation in semi-solid media, in part through a direct affect on isolated B220⁺ pre-B cells. IFN-γ did not induce apoptosis in small resting pre-B cells in BALB/c bone marrow. There was no difference in the proportion of apoptotic B220⁺ pre-B cells in IFN-γ-treated cultures compared to cultures treated with IL-7 alone. However, IL-7-responsive pre-B cells generated from bone marrow had a 30-50% loss in cells in S+G₂/M phases of the cell cycle and an increase of up to twice as many in apoptotic cells within 48 hr of exposure to IFN-γ. Notably, expression of the tyrosine phosphatase B220 was increased in the IFN-γ-treated pre-B cells. Interestingly, although there was no substantial change in IL-7 receptor mRNA expression upon IFN-γ treatment, a small decrease in binding of biotinylated IL-7 to IFN-γ-treated pre-B cells was observed. These results suggest that IFN-γ inhibits IL-7 responsiveness in pre-B cells, resulting in a subtle down-regulation of IL-7 binding, inhibition of proliferation and, ultimately, apoptosis.