ABSTRACT
Gene knockout experiments in mice have suggested a hierarchical model of early B cell commitment wherein E2A proteins (E47 and E12) activate early B cell factor (Ebf1), which in turn activates expression of the B cell commitment factor, Pax5. In IL-7 receptor alpha (IL-7Ralpha) knockout mice, B cell development is blocked before B-lineage commitment at the prepro-B cell stage in adult animals. In IL-7Ralpha(-/-) prepro-B cells, E47 is expressed and yet is insufficient to transcriptionally activate the putative downstream target gene, Ebf1. In this study, we show that further increases of E47 expression in IL-7Ralpha(-/-) prepro-B cells fails to activate Ebf1, but rather leads to a dramatic induction of the E2A inhibitory factors, Id2 and Id3. In contrast, enforced expression of Ebf1 in IL-7Ralpha(-/-) bone marrow potently down-regulates Id2 and Id3 mRNA expression and restores B cell differentiation in vivo. Down-regulation of both Id2 and Id3 during B cell specification is essential in that overexpression of either Id2 or Id3 in wild-type bone marrow blocks B cell specification at the prepro-B cell stage. Collectively, these studies suggest a model where Ebf1 induction specifies the B cell fate by dramatically increasing activity of E47 at the posttranslational level.
Subject(s)
B-Lymphocytes/cytology , Down-Regulation , Inhibitor of Differentiation Protein 2/genetics , Inhibitor of Differentiation Proteins/genetics , Receptors, Interleukin-7/deficiency , TCF Transcription Factors/physiology , Trans-Activators/physiology , Animals , Bone Marrow , Cell Differentiation , Cell Lineage , Gene Expression Regulation , Inhibitor of Differentiation Protein 2/physiology , Inhibitor of Differentiation Proteins/physiology , Interleukin-7/deficiency , Mice , Mice, Knockout , Precursor Cells, B-Lymphoid , RNA, Messenger/analysis , TCF Transcription Factors/genetics , Transcription Factor 7-Like 1 ProteinABSTRACT
The CD4(+) T cell-mediated inflammatory response to Pneumocystis carinii (PC) critically contributes to the clinical severity of PC pneumonia. It has been suggested that lymphopenic conditions predispose individuals to this immunopathology, although the mechanisms remain poorly understood. Another set of evidence indicates that a subpopulation of CD4(+) T cells constitutively expressing the CD25 molecule prevent lymphopenia-induced autoimmunity and inflammatory bowel disease. We tested the ability of this CD25(+)CD4(+) population to regulate CD4(+) T cell-mediated inflammatory response to PC. Adoptive transfer of CD25(-)CD4(+) cells into PC-infected recombination-activating gene-2-deficient mice led to lethal pneumonia within 13 days post-transfer. PC infection appeared to trigger CD25(-)CD4(+) cells, since recipients with reduced PC load survived up to 5 weeks after transfer. In contrast, transfer of CD25(+)CD4(+) cells did not induce lethal pneumonia and prevented the development of the disease induced by CD25(-)CD4(+) cells. Furthermore, CD25(-)CD4(+) cells reduced the PC load in the lung, while CD25(+)CD4(+) cells suppressed this immune response. Our results indicate an essential role for CD25(+)CD4(+) T cells in the control of PC-driven immunopathology, and suggest that in immunocompromised hosts PC pneumonia may result from a deficiency in regulatory T cells.