The membrane glycoprotein CD4 enhances antigen-mediated activation of T cells restricted by class II molecules of the major histocompatibility complex (MHC). This positive function has been attributed to the protein tyrosine kinase p56lck (ref. 4), which is noncovalently associated with the cytoplasmic portion of CD4, and is activated on CD4 aggregation. Antigen presentation by MHC class II molecules coaggregates CD4 and the T-cell antigen receptor (TCR alpha beta-CD3). Thus, the mutual specificity of CD4 and TCR alpha beta for the MHC-antigen complex results in the juxtaposition of p56lck and TCR alpha beta-CD3. In contrast, anti-CD4 antibodies can abrogate antigen-induced, as well as anti-TCR-induced T-cell activation, indicating that CD4 might also transduce negative signals. The molecular basis for this opposing function remains unclear. Here we show that the CD4-p56lck complex prohibits the induction of activation signals through the TCR-CD3 complex when not specifically included in the signalling process. This negative effect does not require anti-CD4 treatment, indicating that the induction of distinct negative signals is probably not involved. Rather, the results demonstrate that the CD4-p56lck complex provides prerequisite signals for antigen-receptor-induced T-cell growth and thus characterize a molecular mechanism for functional constraints imposed on T-cell activation by the MHC.

To understand the mechanism(s) by which p56kk participates in T-cell receptor (TCR) signalling, we have examined the effects of mutations in known regulatory domains of p56kk on the ability of F505 p561k to enhance the responsiveness of an antigen-specific murine T-cell hybridoma. A mutation of the amino-terminal site of myristylation (glycine 2), which prevents stable association of p56kk with the plasma membrane, completely abolished the ability of F505 p56kk to enhance TCR-induced tyrosine protein phosphorylation. Alteration of the major site of in vitro autophosphorylation, tyrosine 394, to phenylalanine diminished the enhancement of TCR-induced tyrosine protein phosphorylation by F505 p56kk. Such a finding is consistent with the previous demonstration that this site is required for full activation of p56"k by mutation of tyrosine 505. Strikingly, deletion of the noncatalytic Src homology domain 2, but not of the Src homology domain 3, markedly reduced the improvement of TCR-induced tyrosine protein phosphorylation by F505 Lck. Additional studies revealed that all the mutations tested, including deletion of the Src homology 3 region, abrogated the enhancement of antigen-triggered interleukin-2 production by F505 p561ck, thus implying more stringent requirements for augmentation of antigen responsiveness by F505 Lck. Finally, it was also observed that expression of F505 p56kk greatly increased TCR-induced tyrosine phosphorylation of phospholipase C-yl, raising the possibility that phospholipase C-yl may be a substrate for p56kk in T lymphocytes. Our results indicate that p56kIregulates T-cell antigen receptor signalling through a complex process requiring multiple distinct structural domains of the protein.Evidence that tyrosine protein phosphorylation plays a critical role in T-lymphocyte activation is increasing (for reviews, see references 20 and 49). Indeed, tyrosine phosphorylation of a limited number of partially characterized substrates occurs rapidly after stimulation of the T-cell receptor (TCR) for antigen by either antigen and major histocompatibility complex (MHC) or anti-TCR antibodies (17). This biochemical signal is critical for T-lymphocyte activation, as supported by the finding that tyrosine-specific protein kinase inhibitors (such as genistein or herbimycin A) prevent TCR-induced phosphatidylinositol (PI) turnover, increase in intracellular calcium, and lymphokine release (18,30,40,43). As the TCR complex does not possess an intrinsic tyrosine protein kinase activity, it has been postulated that this process is mediated through the recruitment of membrane-associated tyrosine protein kinases.The Src family of internal membrane tyrosine protein kinases comprises eight well-characterized members named c-Src, c-Fgr, c-Yes, Lck, Fyn, Hck, Lyn, and Blk (for a review, see reference 14). The members of this family have highly conserved structural features including (from the amino terminus to the carboxy terminus) (i) an aminoterminal glycine residue (glycine 2), which is required for myristylation...

Objectives: Values are intrinsic to the use of health technology assessments (HTAs) in health policy, but neglecting value assumptions in HTA makes their results appear more robust or normatively neutral than may be the case. Results of a 2003 survey by the International Network of Agencies for Health Technology Assessment (INAHTA) revealed the existence of disparate methods for making values and ethical issues explicit when conducting HTA.Methods: An Ethics Working Group, with representation from sixteen agencies, was established to develop a framework for addressing ethical issues in HTA. Using an iterative approach, with email exchanges and face-to-face workshops, a report on Handling Ethical Issues was produced.Results: This study describes the development process and the agreed upon framework for reflexive ethical analysis that aims to uncover and explore the ethical implications of technologies through an integrated, context-sensitive approach and situates the proposed framework within previous work in the development of ethics analysis in HTA.Conclusions: It is important that methodological approaches to address ethical reflection in HTA be integrative and context sensitive. The question-based approach described and recommended here is meant to elicit this type of reflection in a way that can be used by HTA agencies. The questions proposed are considered only as a starting point for handling ethics issues, but their use would represent a significant improvement over much of the existing practice.

Prenatal diagnosis of chromosomal disorders has been performed for more than 20 years, mainly for advanced maternal age. Chromosomal abnormality rates derived from second trimester amniocentesis have mainly come from a collection of small-scale studies from North America and Western Europe. Accurate risk estimates for chromosomal abnormalities are important tools for the physician or obstetrician who would need to make referrals to a prenatal genetic center. This paper presents amniocentesis rates of clinically significant cytogenetic abnormalities for various indications, including advanced maternal age, previous chromosomal abnormality, parental structural rearrangement and a family history of aneuploidy as defined in the text. These data come from a Canadian prenatal diagnosis laboratory with more than 20 years experience in second trimester cytogenetic analysis. They show that the overall frequency of chromosomal abnormalities for advanced maternal age (> or = 35 years) is 1.79%. In this group, 21% of all abnormalities are structural rearrangements (including markers) and less than half of all abnormalities are trisomy 21. The advanced maternal age specific risk of aneuploidies at second trimester is 1.24%. Recurrence risk for aneuploidy after a previous one is 1.29%. However, it is much higher (4.84%) for women of > or = 35 years. When a parent's brother, sister, nephew or niece is affected, the risk of occurrence of aneuploidies (0.24%) is not elevated. When there is a balanced translocation in one of the parents, the overall risk is 10.2% for unbalanced translocations and 37.3% for balanced translocations.

To understand the mechanism(s) by which p56lck participates in T-cell receptor (TCR) signalling, we have examined the effects of mutations in known regulatory domains of p56lck on the ability of F505 p56lck to enhance the responsiveness of an antigen-specific murine T-cell hybridoma. A mutation of the amino-terminal site of myristylation (glycine 2), which prevents stable association of p56lck with the plasma membrane, completely abolished the ability of F505 p56lck to enhance TCR-induced tyrosine protein phosphorylation. Alteration of the major site of in vitro autophosphorylation, tyrosine 394, to phenylalanine diminished the enhancement of TCR-induced tyrosine protein phosphorylation by F505 p56lck. Such a finding is consistent with the previous demonstration that this site is required for full activation of p56lck by mutation of tyrosine 505. Strikingly, deletion of the noncatalytic Src homology domain 2, but not of the Src homology domain 3, markedly reduced the improvement of TCR-induced tyrosine protein phosphorylation by F505 Lck. Additional studies revealed that all the mutations tested, including deletion of the Src homology 3 region, abrogated the enhancement of antigen-triggered interleukin-2 production by F505 p56lck, thus implying more stringent requirements for augmentation of antigen responsiveness by F505 Lck. Finally, it was also observed that expression of F505 p56lck greatly increased TCR-induced tyrosine phosphorylation of phospholipase C-gamma 1, raising the possibility that phospholipase C-gamma 1 may be a substrate for p56lck in T lymphocytes. Our results indicate that p56lck regulates T-cell antigen receptor signalling through a complex process requiring multiple distinct structural domains of the protein.