T lymphocytes can recognize and be activated by a very small number of complexes of peptide with major histocompatibility complex (MHC) molecules displayed on the surface of antigen-presenting cells (APCs). The interaction between the T-cell receptor (TCR) and its ligand has low affinity and high off-rate. Both findings suggest that an extremely small number of TCRs must be engaged in interaction with APCs and raise the question of how so few receptors can transduce an activation signal. Here we show that a small number of peptide-MHC complexes can achieve a high TCR occupancy, because a single complex can serially engage and trigger up to approximately 200 TCRs. Furthermore, TCR occupancy is proportional to the T cell's biological response. Our findings suggest that the low affinity of the TCR can be instrumental in enabling a small number of antigenic complexes to be detected.

SummaryWe have investigated the level of TCR occupancy required to elicit different biological responses in human CTL clones specific for an influenza matrix peptide. Specific cytotoxicity could be detected at extremely low peptide concentrations (10 -t2 to 10 -15 M). However, IFN-',/production, responsiveness to IL-2 and Ca ++ fluxes were observed only at peptide concentrations > 10 -9 M, while autonomous proliferation required even higher peptide concentrations. In parallel experiments we measured TCR downregulation to estimate the number of TCRs triggered. We observed that at low peptide concentrations, where only cytotoxicity is triggered, TCI( downregulation was hardly detectable. Conversely, induction of IFN-~/production and proliferation required triggering of at least 20-50% of TCRs.Taken together these results indicate that a single CTL can graduate different biological responses as a function of antigen concentration and that killing of the specific target does not necessarily result in full activation. It is well known that naive CD8 § T cells and mature CTL have different requirements for activation. On the one hand it has been shown that naive T cells require high antigen concentrations (1-3), professional APCs (4, 5) and T cell help (1, 6-8) in order to proliferate and mature into effector cells. On the other hand, mature CTL can kill any target cell displaying very low numbers of peptide-MHC complexes (9), even lower than that required to trigger Th cells (10, 11). These findings have generally been interpreted as evidence that the responsiveness of CTL to antigen is increased after activation and that CTL are far more sensitive than Th cells.We have recently demonstrated that TCP-, downregulanon can be used to measure the number of TCRs triggered at the level of T-APC interaction. Using this assay we observed that in Th clones activation to IFN-~ production requires that ~30% of the TCt(s are triggered (12). Here we investigated the relationship between antigen concentration, TCR occupancy, and the induction of different biological responses in CTL. We found that in human CTL clones, cytotoxicity can be elicited by triggering a very small number of TCt(s, in conditions where no other responses can be detected. At increasing levels of TCR. occupancy, helper-dependent and -independent proliferation and IFN-~/production were observed as a function of the number of TCRs triggered. These results demonstrate that different responses can be elicited in cytotoxic T lymphocytes by different levels of TCR occupancy. Materials and MethodsT Cell Clones and Target Cells. Clones speczfic for the influenza matrix peptide 58-66 (M58-66, reference 13) were isolated from polyclonal cell lines derived from two different donors and maintamed as described (14). The clones were used 10-30 d after restimulation. An HLA-A2 + EBV-transformed B cell line 0"Y) was used as antigen presenting/target cell.Cytotoxicity. Cytotoxicity was measured in a standard 4 h 51Cr release assay using 5,000 target cells/well and different E...

No abstract

Rehabilitation of locomotion in spinal cord (s.c.) injured patients is unsatisfactory. Here we report the effects of a novel 'Laufband (LB; treadmill) therapy' based on 'rules of spinal locomotion' derived from lower vertebrates. Eighty-nine incompletely paralysed (44 chronic and 45 acute) para- and tetraplegics underwent this therapy, then were compared with 64 patients (24 chronic and 40 acute) treated conventionally. The programme consisted of daily upright walking on a motor driven LB initially with body weight support (BWS) provided by a harness and assisted limb movements by the therapists when necessary. Forty-four chronic patients with different degrees of paralysis undertook the programme for 3-20 weeks (median = 10.5), 0.5-18 years after s.c. damage. At the onset of LB therapy 33/44 patients were wheelchair-bound (no standing and/or walking without help by others) whereas at the end of therapy 25 patients (76%) had learned to walk independently, 7 patients with help [corrected]. Only 1 subject did not improve. It was striking that voluntary muscle activity in the resting position was still low in several patients who had gained walking capability. Eleven patients who could already walk before LB therapy improved in speed and endurance. Of the 44 patients, six were capable of staircase walking before LB therapy compared with 34 afterwards. In order to validate the apparent superiority of LB therapy two types of comparisons were performed. In a 'temporal' control 12 spastic paretic patients, still wheelchair-bound after the period of postacute conventional therapy, performed LB immediately thereafter. After completion of LB therapy nine of these patients had learned to walk without help from others.(ABSTRACT TRUNCATED AT 250 WORDS)

SUMMARY It has long been thought that clonal deletion efficiently removes almost all self-specific T cells from the peripheral repertoire. But here we found that self peptide-MHC specific CD8+ T cells in the blood of healthy humans were present in frequencies similar to those specific for non-self antigens. For the Y chromosome encoded SMCY antigen, self-specific T cells exhibited only a three-fold lower average frequency in males versus females and were anergic with respect to peptide activation, although this inhibition could be overcome by a stronger stimulus. We conclude that clonal deletion prunes but does not eliminate self-specific T cells and suggest that to do so would create holes in the repertoire that pathogens could readily exploit. In support of this hypothesis, we detected T cells specific for all 20 amino acid variants at the p5 position of a hepatitis C virus epitope in a random group of blood donors.