An Important Role for CD1d and NKT Cells in the Suppression of Hematopoiesis in Mice Induced by Infection with Cytomegalovirus.

Cytomegalovirus (CMV) is a serious problem in the context of immunocompromised hosts after conditioning for stem cell transplantation. Infection with CMV impacts hematopoiesis, and inhibits colony formation by myeloid progenitor cells (MPC) in vitro and myelopoiesis in vivo. It is not clear how these effects are mediated. Murine (M)CMV has been used as a model for the study of viral pathogenesis and persistent infections. Little is known how CD1d or Natural Killer T (NKT) cells influence effects of CMV infections on hematopoiesis. We hypothesized that CD1d and NKT cells may play a protective role in this effect. We first assessed a role for CD1d on hematopoiesis and found that absolute numbers of MPC (CFU-GM, BFU-E, CFU-GEMM) in marrow and spleen of CD1d −/− mice were significantly higher than in control mice, an observation reproducible in CD1d −/− mice on either a C57Bl/6 or Balb/c background, suggesting that the effects were not due to mouse strain differences. This increased number of MPC was due to increased proliferation of MPC in marrow and spleen since the percentage of MPC in S phase of the cell cycle in CD1d −/− mice, as determined by a high specific activity tritiated thymidine kill assay, was at least two fold higher than in control (+/+) mice. CD1d −/− mice are deficient in NKT cells. We also used Jα18 −/− (C57Bl/6 and Balb/c) mice, which are deficient in NKT cells but express normal cell surface levels of CD1d expression, to demonstrate that the enhanced hematopoiesis in CD1d −/− mice was not a reflection of NKT cell effects; Jα18 −/− mice had normal numbers and cycling status of MPC in marrow and spleen. We then assessed the effects of MCMV (5 x 10⁴ plaque forming units per mouse) on absolute numbers and cycling of MPC in marrow and spleen of +/+, CD1d −/−, and Jα18 −/− mice. Mice were evaluated 4 days post MCMV infection when viral titers are high. While MCMV had some suppressive effects on MPC numbers and cycling of MPC in +/+ C57Bl/6 and Balb/c mice, this suppression was much more pronounced in CD1d −/− and Jα18 −/− mice, with the most potent MCMV effects manifested on these −/− mice on a Balb/c background strain, suggesting that lack of CD1d and/or NKT cells are involved and likely protective in these suppressive viral effects on hematopoiesis. In order to confirm that NKT cells were responsible for this MCMV-induced decrease in hematopoiesis, we utilized Jα18 −/− mice on a Balb/c background to evaluate effects of adding back to these mice (which don’t contain NKT cells) purified Vα 14 Jα18 T cell receptor positive cells isolated from murine liver using CD1d dimers loaded with αGalCer. Mice were given these NKT cells at 2.5 x 10⁵ cells i.v./mouse and immediately after injected with MCMV i.p. The NKT cells blocked MCMV-induced suppression of absolute numbers and cycling of marrow and spleen CFU-GM, BFU-E, and CFU-GEMM, demonstrating that NKT cells play a role in and can block MCMV effects on murine hematopoiesis. This information may thus be relevant to treatment of CMV infections during stem cell transplantation.