Whether thymocytes adopt an αβ or a γδ T cell fate in the thymus is determined at a checkpoint (β-selection) by the relatively weak or strong signals that are delivered by either the pre-T cell receptor (preTCR) or the γδ TCR, respectively. However, how these signals are initiated, and how different signal strengths are generated, remains unclear. Although binding of thymic agonist ligand would predict strong signaling, the preTCR and TCRγδ appear to be capable of ligandindependent signaling. Some reports have suggested that receptor oligomerization, which is thought to be mediated by either the immunoglobulin (Ig)-like domain of the preTCR α-chain (pTα) or the variable domain of TCRδ, is a unifying mechanism that initiates signaling in early CD4 − CD8 − double negative (DN) thymocyte progenitors. Here, we demonstrate that the extracellular regions of pTα and TCRd that were implicated in mediating receptor oligomerization were not required for signal initiation from the preTCR or TCRγδ. Indeed, a truncated TCRγδ that lacked all of its extracellular Ig-like domains still formed a signaling-competent TCR that drove cells through the β-selection checkpoint. These observations suggest that signal initiation in DN thymocytes is simply a consequence of the surface-pairing of TCR chains, with signal strength being a function of the abundances of surface TCR. Thus, processes that regulate the surface abundances of TCR complexes in DN cells, such as oligomerization-induced endocytosis, would be predicted to have a major influence in determining whether cells adopt an αβ versus γδ T cell fate.

Three commonly used Mycobacterium bovis BCG vaccine strains elicited different magnitudes of T-cell activation and gamma interferon production in vitro in healthy BCG-vaccinated individuals. Glaxo 1077 exhibited the greatest stimulatory capacity, followed by Pasteur 1173 and then Danish 1331. These differences may affect in vitro stimulation and vaccination-induced immunogenicity.Mycobacterium bovis bacillus Calmette-Guérin (BCG) is the only vaccine currently available to prevent tuberculosis. It is administered to over 100 million people per year (25), although its efficacy against pulmonary TB worldwide is variable (11). M. bovis BCG is an attenuated form of M. bovis obtained by 13 years of serial passage in vitro, following which it was found to provide protective immunity to challenge with virulent M. tuberculosis (3). M. bovis BCG was maintained by a process of serial passage for 40 years before methods for stable long-term storage became available, and this culminated in a family of approximately 13 daughter strains which are collectively known today by the generic term BCG. These strains have been shown to differ genetically (2,5,6,22) Despite the numerous differences previously found among BCG strains, and despite BCG stimulation being routinely used to study mycobacterium-specific immune responses, Tcell responses elicited by different BCG strains have not been compared in human cultures and were therefore investigated in this study.The BCG vaccine strains Danish 1331, Pasteur 1173, and Glaxo 1077 were grown in liquid cultures of Middlebrook 7H9 medium (Difco) supplemented with 10% Bacto Middlebrook ADC (Scientific Laboratory Supplies) and 0.025% Tween 80 (Difco), quantified by growth on Middlebrook 7H10 agar plates for 3 weeks, and kept frozen at Ϫ80°C for long-term storage. All three strain preparations were found to be 90 to 95% viable, as determined by fluorescein diacetate/ethidium bromide labeling. Peripheral blood mononuclear cells (PBMC) were isolated by density gradient centrifugation from seven healthy BCG-vaccinated donors, at least five of whom were likely to have been vaccinated with Glaxo 1077, and were cultured for 6 days either in the absence of antigenic stimulation or in the presence of one of the live M. bovis BCG strains titrated between 0.01 and 1 CFU per PBMC: if monocytes are estimated to constitute 10% of total PBMC, this is the equivalent of multiplicity of infection (MOI) ratios (M. bovis BCG bacilli to monocytes) of 0