Antigen persistence and time of T-cell tolerization determine the efficacy of tolerization protocols for prevention of skin graft rejection.

We studied antigen-specific T-cell tolerization therapy using skin transplantation across a defined minor histocompatibility antigen difference. Specific tolerization protocols using short-lived peptide or long-lived spleen cells presenting the peptide as antigen prevented graft rejection without immunosuppression when started before or as long as 10 days after transplantation. Peptide-induced T-cell tolerance was transient, and antigen presentation by the graft was not sufficient to maintain tolerance. In contrast, transfer of antigen-expressing lymphoid cells induced long-lasting tolerance correlating with donor cell chimerism. These findings show that antigen-specific tolerization can induce graft acceptance even when begun after transplantation and that long-term graft survival depends on persistence of the tolerizing antigen.

Beyond double-stranded RNA-type I IFN induction by 3pRNA and other viral nucleic acids.

Production of type I IFN is the key response to viral infection. Since the discovery of type I IFNs in 1957, long double-stranded RNA formed during replication of many viruses was thought to be responsible for type I IFN induction, and for decades double-stranded RNA-activated protein kinase (PKR) was thought to be the receptor. Recently, this picture has dramatically changed. It now became evident that not PKR but two members of the Toll-like receptor (TLR) family, TLR7 and TLR9, and two cytosolic helicases, RIG-I and MDA-5, are responsible for the majority of type I IFNs induced upon recognition of viral nucleic acids. In this review, we focus on the molecular mechanisms by which those innate immune receptors detect viral infection. Based on the recent progress in the field, we now know that TLR7, TLR9, and RIG-I do not require long double-stranded RNA for type I IFN induction.

Plants as sources of airborne bacteria, including ice nucleation-active bacteria.

Vertical wind shear and concentration gradients of viable, airborne bacteria were used to calculate the upward flux of viable cells above bare soil and canopies of several crops. Concentrations at soil or canopy height varied from 46 colony-forming units per m over young corn and wet soil to 663 colony-forming units per m over dry soil and 6,500 colony-forming units per m over a closed wheat canopy. In simultaneous samples, concentrations of viable bacteria in the air 10 m inside an alfalfa field were fourfold higher than those over a field with dry, bare soil immediately upwind. The upward flux of viable bacteria over alfalfa was three- to fourfold greater than over dry soil. Concentrations of ice nucleation-active bacteria were higher over plants than over soil. Thus, plant canopies may constitute a major source of bacteria, including ice nucleation-active bacteria, in the air.

Relationship between Ice Nucleation Frequency of Bacteria and Frost Injury.

Not every cell of a given bacterial isolate that has ice-nucleating properties can serve as an ice nucleus at any given time and temperature. The ratio between the number of ice nuclei and number of bacterial cells in a culture (i.e. nucleation frequency) was found to vary with incubation temperature, growth medium composition, culture age, and genotype. Optimal conditions for ice nucleus production in vitro included incubation of the bacterial cells at 20 to 24 degrees C on nutrient agar containing glycerol. The relationship between nucleation frequency and frost injury was examined by subjecting corn seedlings to -4 degrees C immediately after they were sprayed with bacterial suspensions with different nucleation frequencies and by following both ice nucleus concentration and bacterial population size on leaves of corn seedlings as a function of time after bacterial application. The amount of frost injury to growth chamber-grown corn seedlings at -4 degrees C was a function of the number of ice nuclei active at that temperature on the leaves. The number of ice nuclei, in turn, is the product of the nucleation frequency and population size of ice-nucleation-active bacteria present on the leaves.

Donor cell persistence and activation-induced unresponsiveness of peripheral CD8+ T cells.

We studied the impact of the duration of donor cell persistence on CD8+ T cell responsiveness after adoptive transfer of antigen-expressing lymphoid cells. Naive or immunized female mice were treated by adoptive transfer of spleen cells from mice ubiquitously expressing a lymphocytic choriomeningitis virus-derived cytotoxic T lymphocyte (CTL) epitope (gp33-41) either alone or in combination with the male H-Y antigen providing additional antigenic CTL and T helper cell determinants. Low doses of male spleen cells (or sorted B cells) primed CTL, while high doses of the same cells rendered them unresponsive. CTL unresponsiveness induced by high numbers of male spleen cells was dependent upon prolonged persistence of antigen-expressing donor cells. Unresponsive CTL reverted to a state of activation when the duration of donor cell chimerism was limited. Memory CTL could be rendered unresponsive if antigen-expressing donor cells were allowed to persist. These results suggest that, irrespective of the type of antigen-presenting cell and the functional state of the responding T cell, activation and unresponsiveness can represent two different outcomes critically determined by quantitative and kinetic differences of antigen persistence.

Direct quantitation of rapid elimination of viral antigen-positive lymphocytes by antiviral CD8(+) T cells in vivo.

Lysis of infected cells by CD8(+) T cells is an important mechanism for the control of virus infections, but remains difficult to quantify in vivo. Here, we study the elimination kinetics of viral antigen-positive lymphocytes by antiviral CD8(+) T cells using flow cytometry and mathematical analysis. In mice acutely infected with lymphocytic choriomeningitis virus, more than 99.99 % of target cells were eliminated each day, corresponding to a half-life of 1.4 h. Even in mice exposed to virus 300 days previously, and with no ex vivo killing activity, 84 % of the target cells were eliminated per day. Unexpectedly, the elimination kinetics of antigen-positive lymphocytes was not significantly impaired in mice deficient in either perforin-, CD95 ligand- or TNF-mediated cytotoxicity. For viruses with a particular tropism for lymphocytes, such as Epstein-Barr virus or HIV, our results illustrate how effectively CD8(+) T cell-mediated elimination of target cells can potentially contribute to virus control and immunosuppression.