STAT6-deficient mice exhibit normal induction of murine AIDS and expression of immunoglobulin E following infection with LP-BM5 murine leukemia viruses

Abstract

The unique Gag polyprotein of the replication-defective virus responsible for murine AIDS (MAIDS) induces B-cell activation, proliferation, and differentiation, including immunoglobulin class switch-recombination to immunoglobulin E (IgE). Secretion of IgE normally requires the serial induction of interleukin 4 (IL-4), engagement of the IL-4 receptor, activation of signal transducer and activator of transcription (STAT) 6, and induction of Iepsilon germline transcripts as a prelude to switching. Remarkably, expression of IgE is equivalent in normal and IL-4-deficient mice with MAIDS (Morawetz et al., J. Exp. Med. 184:1651-1661, 1996). To understand this anomaly, we studied mice with a null mutation of STAT6. Lymphoproliferation and immunodeficiency, the hallmarks of MAIDS, developed with comparable kinetics and degree in normal and mutant mice. In addition, serum IgE levels were indistinguishable in mice of either genotype. We conclude that B cells from mice with MAIDS activate unique IL-4- and STAT6-independent signaling pathways for B-cell activation and differentiation.

FIG. 1

FACS analyses of spleen cells from mice infected with LP-BM5 viruses for 8 weeks. Cells were stained with the indicated antibodies to cell surface antigens, and 2 × 10⁵ viable cells, as determined by size and exclusion of propidium iodide, were evaluated. Similar results were obtained with mice infected for 8 to 12 weeks. FSC, forward angle scatter.

FIG. 2

Proliferative responses of spleen cells to stimulation with mitogens or PMA plus ionomycin. Cells (2 × 10⁵) from spleens of uninfected mice or mice infected for 8 weeks were cultured for 48 h in triplicate in 96-well plates with ConA (2 μg/ml), LPS (20 μg/ml), or PMA (20 ng/ml) plus ionomycin (250 ng/ml). Cultures were pulsed with [³H]thymidine for the last 6 h of culture. Data are average values ± 1 standard error of the mean for three mice per time point. Similar results were obtained in a second experiment. CPM, counts per minute.

FIG. 3

IgE levels in serum in mice infected with LP-BM5 MuLV. Sera obtained at the indicated time points were assayed for IgE levels by ELISA. Data are the averages ± 1 standard error of the mean for three mice per time point.