Class II transactivator-mediated regulation of major histocompatibility complex class II antigen expression is important for hematopoietic progenitor cell suppression by chemokines and iron-binding proteins.

OBJECTIVE: Iron-binding proteins H-ferritin (HF) and lactoferrin (LF), as well as chemokines, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma suppress hematopoietic progenitor cell (HPC) proliferation. Major histocompatibility complex (MHC) class II antigens have been associated with suppressive effects of HF and LF. Because the transcription factor class II transactivator (CIITA) regulates expression of MHC class II antigens, we evaluated influences of CIITA and MHC class II antigens on suppression of colony formation by murine bone marrow HPC in response to HF, LF, CC, and CXC chemokines, TNF-alpha, and IFN-gamma. We also evaluated hematopoiesis in mice deficient in both CIITA and MHC class II antigens (CIITA -/-), in mice deficient in MHC class II antigens but not in CIITA (MHC class II -/-), and in mice deficient in CIITA but not in MHC class II antigens (CIITA-IE).

MATERIALS AND METHODS: HF, LF, CCL3/MIP-1alpha, CXCL5/ENA-78, CXCL8/IL-8, CCL5/RANTES, TNF-alpha, and IFN-gamma were assessed for effects on colony formation by bone marrow HPC (colony-forming unit granulocyte-macrophage, burst-forming unit erythroid, and colony-forming unit multipotential) stimulated in vitro by combinations of growth factors including erythropoietin, stem cell factor, pokeweed mitogen mouse spleen cell conditioned medium, and hemin. Bone marrow cells were from CIITA -/-, MHC class II antigen -/-, CIITA-IE, and littermate control mice. We also evaluated cycling status (percent cells in S-phase) and absolute numbers of marrow and spleen HPC in these mice.

RESULTS: Multiple growth factor-stimulated colony formation by control bone marrow HPC was significantly suppressed by HF, LF, CCL3, CXCL5, CXCL8, TNF-alpha, and IFN-gamma, but not by CCL5. However, HPC from CIITA -/- and MHC class II antigen -/- mouse marrow was insensitive to inhibition by HF, LF, CCL3, CXCL5, CXCL8, and CCL5; these HPC were inhibited by TNF-alpha and IFN-gamma. Restoration of MHC class II expression in CIITA -/- (CIITA-IE) mice restored responsiveness of HPC to inhibition by HF, LF, CCL3, CXCL5, and CXCL8. Increased cycling of splenic HPC in CIITA -/- and MHC class II antigen -/-, compared to control and CIITA-IE, mice was noted.

CONCLUSIONS: Myelosuppressive effects of iron-binding proteins HF and LF and chemokines CCL3, CXCL5, and CXCL8 on mouse bone marrow HPC require expression of MHC class II antigens, and CIITA is involved in this responsiveness through its regulation of expression of MHC class II antigens.