The transforming growth factor ,Bs (TGF-,Bs) are a group of multifunctional growth factors which inhibit cell cycle progression in many cell types. The TGF-,pinduced cell cycle arrest has been partially attributed to the regulatory effects of TGF-j8 on both the levels and the activities of the G1 cyclins and their kinase partners. The activities of these kinases are negatively regulated by a number of small proteins, p21 (WAFi, Cipl), p27KiPl, p16, and p15INK4B, that physically associate with cyclins, cyclindependent kinases, or cyclin-Cdk complexes. p21 has been previously shown to be transcriptionally induced by DNA damage through p53 as a mediator. We demonstrate that TGF-13 also causes a rapid transcriptional induction of p21, suggesting that p21 can respond to both intracellular and extracellular signals for cell cycle arrest. In contrast to DNA damage, however, induction of p21 by TGF-j3 is not dependent on wild-type p53. The cell line studied in these experiments, HaCaT, contains two mutant alleles of p53, which are unable to activate transcription from the p21 promoter when overexpressed. In addition, TGF-j3 and p53 act through distinct elements in the p21 promoter. Taken together, these findings suggest that TGF-j8 can induce p21 through a p53-independent pathway. Previous findings have implicated p27KiP' and p15INK2B as effectors mediating the TGF-f3 growth inhibitory effect. These results demonstrate that a single extracellular antiproliferative signal, TGF-j8, can act through multiple signaling pathways to elicit a growth arrest response.

Antigen (Ag)-driven selection of helper T cells (Th) in normal animals has been difficult to study and remains poorly understood. Using the major histocompatibility complex class II– restricted murine response to pigeon cytochrome c (PCC), we provide evidence for both preimmune and Ag-driven selection in the evolution of Ag-specific immunity in vivo. Before antigenic challenge, most Vα11+Vβ3+ Th (70%) express a critical complementarity-determining region 3 (CDR3) residue (glutamic acid at TCR-α93) associated with PCC peptide contact. Over the first 5 d of the primary response, PCC-responsive Vα11+Vβ3+ Th expressing eight preferred CDR3 features are rapidly selected in vivo. Clonal dominance is further propagated through selective expansion of the PCC-specific cells with T cell receptor (TCR) of the “best fit.” Ag-driven selection is complete before significant emergence of the germinal center reaction. These data argue that thymic selection shapes TCR-α V region bias in the preimmune repertoire; however, Ag itself and the nongerminal center microenvironment drive the selective expansion of clones with preferred TCR that dominate the response to Ag in vivo.

Cowpox virus Brighton red strain (CPV) contains a gene, crmD, which encodes a 320-aa tumor necrosis factor receptor (TNFR) of 44% and 22% identity, respectively, to the CPV TNFR-like proteins, cytokine response modifiers (crm) CrmB and CrmC. The crmD gene was interrupted in three other cowpox strains examined and absent in various other orthopoxviruses; however, four strains of ectromelia virus (ECT) examined contained an intact crmD (97% identity to CPV crmD) and lacked cognates of crmB and crmC. The protein, CrmD, contains a transport signal; a 151-aa cysteinerich region with 21 cysteines that align with human TNFRII ligand-binding region cysteines; and C-terminal region sequences that are highly diverged from cellular TNFR Cterminal region sequences involved in signal transduction. Bacterial maltose-binding proteins containing the CPV or ECT CrmD cysteine-rich region bound TNF and lymphotoxin-␣ (LT␣) and blocked their in vitro cytolytic activity. Secreted viral CrmD bound TNF and LT␣ and was detectable after the early stage of replication, using nonreducing conditions, as 60-to 70-kDa predominant and 90-to 250-kDa minor disulfide-linked complexes that were able to be reduced to a 46-kDa form and deglycosylated to a 38-kDa protein. Cells infected with CPV produced extremely low amounts of CrmD compared with ECT. Possessing up to three TNFRs, including CrmD, which is secreted as disulfide-linked complexes in varied amounts by CPV and ECT, likely enhances the dynamics of the immune modulating mechanisms of orthopoxviruses.