Reconstitution of early lymphoid proliferation and immune function in Jak3-deficient mice by interleukin-3

Expansion of early lymphoid progenitors requires interleukin-7 (IL-7), whic h functions through gamma(c)-mediated receptor activation of Jak3. Jak3 def iciency is a cause of severe combined immunodeficiency (SCID) in humans and mice. IL-3 activates many of the same signaling pathways as IL-7, such as Stat5, but achieves this effect through the activation of Jak2 rather than Jak3. We hypothesized that expansion of an IL-7-responsive precursor popula tion through a Jak3-independent pathway using IL-3 may stimulate early lymp hoid progenitors and restore lymphopoiesis in Jak3(-/-) mice. Newborn Jak3( -/-) mice that were injected with IL-3 demonstrated thymic enlargement, a 2 - to 20-fold increase in thymocyte numbers, and up to a 10-fold expansion i n the number of CD4(+), CD8(+), and B220(+)/IgM(+) splenic lymphocytes, con sistent with an effect upon an early lymphoid progenitor population. In con trast to control mice, IL-3-treated Jak3(-/-) mice challenged with the allo geneic major histocompatibility complex (MHC) class I-bearing tumor P815 de veloped a specific CD8-dependent cytotoxic T lymphocyte (CTL) response. IL- 3-treated mice also mounted influenza-specific CTL responses and survival w as prolonged. The beneficial effects of IL-3 are proposed to be produced by stimulation of a lymphoid precursor population of IL-7R alpha(+)/IL-3R alp ha(+) cells that we identified in wild-type bone marrow. In vitro, we show that an early IL-7R(+) lymphoid progenitor population expresses IL-3R and p roliferates in response to IL-3 and that IL-3 activates Stat5 comparably to IL-7. Clinically, IL-3 may therefore he useful treatment for X-linked and Jak3-deficient SCID patients who lack bone marrow donors. (C) 1999 by The A merican Society of Hematology.