Antigen-specific T helper cell function: differential cytokine expression in primary and memory responses.

Distinguishing between the development of functional potential in antigen-specific T helper (Th) cells and the delivery of these specialized functions in vivo has been difficult to resolve. Here, we quantify the frequency of cytokine-producing cells within the primary and memory B10.BR Th cell response to pigeon cytochrome c (PCC). In vitro analysis of acquired functional potential indicated no Th1/Th2 cytokine polarity at the peak of the primary response with surprisingly little evidence for the selective preservation of interleukin (IL)-2, tumor necrosis factor (TNF)-alpha, IL-4, and interferon (IFN)-gamma potentials into the memory compartment. However, the expression of these functional potentials appears tightly regulated in vivo. The staggered appearance of primary response cytokines directly ex vivo contrasts markedly with their rapid coordinate expression in the memory response. Frequencies of IL-2-, TNF-alpha-, IFN-gamma-, and IL-10-expressing memory responders increased over their primary response counterparts, but were still markedly lower than revealed in vitro. IL-4-, IFN-gamma-, and IL-10-expressing Th cells remained at low but stable frequencies over the first 6 d of the memory response. Analysis of T cell receptor beta chain sequences of IL-4- and TNF-alpha-expressing PCC-specific Th cells provides evidence for early functional commitment among clonal progeny. These data indicate that the development of functional potential is a consequence of initial antigen experience, but delivery of specialized functions is differentially regulated in primary and memory immune responses.

Evolution of antigen-specific T cell receptors in vivo: preimmune and antigen-driven selection of preferred complementarity-determining region 3 (CDR3) motifs.

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 Valpha11(+)Vbeta3(+) Th (70%) express a critical complementarity-determining region 3 (CDR3) residue (glutamic acid at TCR-alpha93) associated with PCC peptide contact. Over the first 5 d of the primary response, PCC-responsive Valpha11(+)Vbeta3(+) 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-alpha 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.

A third distinct tumor necrosis factor receptor of orthopoxviruses.

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 cysteine-rich 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 C-terminal region sequences involved in signal transduction. Bacterial maltose-binding proteins containing the CPV or ECT CrmD cysteine-rich region bound TNF and lymphotoxin-alpha (LTalpha) and blocked their in vitro cytolytic activity. Secreted viral CrmD bound TNF and LTalpha 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.

Transforming growth factor beta induces the cyclin-dependent kinase inhibitor p21 through a p53-independent mechanism.

The transforming growth factor beta s (TGF-beta s) are a group of multifunctional growth factors which inhibit cell cycle progression in many cell types. The TGF-beta-induced cell cycle arrest has been partially attributed to the regulatory effects of TGF-beta 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 (WAF1, Cip1), p27Kip1, p16, and p15INK4B, that physically associate with cyclins, cyclin-dependent 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-beta 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-beta 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-beta and p53 act through distinct elements in the p21 promoter. Taken together, these findings suggest that TGF-beta can induce p21 through a p53-independent pathway. Previous findings have implicated p27Kip1 and p15INK2B as effectors mediating the TGF-beta growth inhibitory effect. These results demonstrate that a single extracellular antiproliferative signal, TGF-beta, can act through multiple signaling pathways to elicit a growth arrest response.