Regulatory mechanisms in cytotoxic T lymphocyte development. II. Dissociation of in vitro cytotoxic T-lymphocyte and suppressor T-cell activities with L-ornithine

Different signaling pathways induced by alpha-CD3 monoclonal antibody versus alloantigen on the basis of differential ornithine sensitivity

Previously we reported that 10 mM ornithine (Orn) selectively inhibits the development of CD8+ CTL in MLC. Herein we show that induction by alpha-CD3 mAb of CD8+ killer cells which manifest antibody-redirected cytotoxicity (ARC) of FcR+ targets is not Orn sensitive. Orn resistance was independent of activation kinetics or alpha-CD3 mAb concentration. alpha-CD3 mAb added to the cytotoxicity assay did not reveal a cytolytic potential in CTL from an Orn-treated MLC when the target cells bore both the inducing alloantigen and FcR. Addition of alpha-CD3 mAb to MLC failed to overcome Orn inhibition of CTL and yet induced ARC activity in the same culture. Expression of mRNA for pore-forming proteins (PFP) and granzyme B was inhibited by Orn in CTL but not in ARC killer cells. Our results demonstrate differences in the T cell activation process stimulated by alloantigen or alpha-CD3 mAb.

Differential sensitivity of cytotoxic T lymphocytes and lymphokine-activated killer cells to inhibition by L-ornithine

The selective inhibition of murine cytotoxic T lymphocyte (CTL) differentiation in C57B1/6 (B6) anti-DBA/2 mixed leukocyte cultures (MLC) by the amino acid L-ornithine (Orn) could not be reversed by addition of up to 1000 U/ml IL-2. Analysis of the effects of Orn on induction of lymphokine-activated killer (LAK cells), using dosages of IL-2 from 10-1000 U/ml and measuring cytolytic activity against two tumor targets (P815 and YAC-1) over the course of 5 days, indicated that LAK cells were not suppressed by Orn. LAK precursors and effector cells were CD8- and ASGM1+, indicating that they were derived from natural killer (NK) cells. We also found that the growth and maintenance of cloned CTL lines were not sensitive to inhibition by Orn; nor was their acquisition of nonspecific cytolytic activity in the presence of high lymphokine concentrations. Thus, induction of naive CTL shows differential susceptibility to Orn inhibition relative to LAK and LAK-like activities by NK and cloned CTL lines in response to IL-2.

Phenotypic and functional heterogeneity of thymic and splenic lymphokine activated killer cells relative to ornithine sensitivity

We have previously reported the selective inhibition of cytotoxic T lymphocytes (CTL) by 10 mM ornithine (ORN) relative to natural killer (NK) cell-derived lymphokine activated killer cells (LAK). To determine if this were due to differences in the progenitor cells or the type of stimulus, we used cortisone-resistant thymocytes (CRT) as a source of mature T cells for induction of LAK and CTL, and compared the results with spleen. Thymic and splenic CTL precursors (CTLp) from C57B1/6 (B6) mice were CD8+, ASGM1-, ORN sensitive. Splenic LAK precursors (LAKp) were CD8-, ASGM1+, ORN resistant when assayed against both YAC-1 and P815 tumor targets. In contrast, CRT-derived LAKp were CD8-, ASGM1+, ORN resistant against YAC-1, whereas LAKp against P815 were CD8+, ASGM1+, ORN sensitive. ORN sensitivity was also observed among CTL and LAK in DBA/2 mice and was associated with CD8+ phenotype. Therefore, our initial observation of differential ORN sensitivity in CTL vs LAK was a function of the progenitor cells; furthermore, CD8+ cytolytic cells are ORN sensitive whether activated by antigen (CTL) or IL-2 (T-LAK).

Immunosuppressive activity of staphylococcal enterotoxin B. I. Characterization of staphylococcal enterotoxin-B-induced suppressor cells

Staphylococcal enterotoxin B (SEB) binds specifically to major histocompatibility complex class II molecules on the surface of accessory cells and stimulates virtually all T cells bearing certain, but not all, T cell-receptor V beta alleles. We have previously shown that this superantigen is a potent inducer of multiple regulatory T cell populations. In the present report we show that SEB induces a population of suppressor T cells which inhibits the generation of alloantigen-induced cytotoxic T cell activity. Using both negative- and positive-selection analysis, we found that this suppressor population is a CD4- CD8- CD5+ IL-2R+ T cell. This cell population inhibited both syngeneic and allogeneic cytotoxic T lymphocyte activity, but the cell population which inhibited allogeneic CTL activity was radiation sensitive. In addition, allogeneic SEB-primed cells appeared to develop cytolytic activity as a result of the additional stimulation in the mixed-lymphocyte reaction culture. The relationship of the SEB-primed CD4- CD8- CD5+ T cells to related regulatory T cell populations is discussed.

Cytotoxic T lymphocytes are the prime mediators of suppression of the mixed lymphocyte reaction by alloactivated cells

An alloactivated secondary mixed lymphocyte reaction (MLR) culture (SMC), which was suppressive in the MLR with autologous responder cells, was studied in more detail. In particular, we investigated whether the suppressive activity of this SMC was mediated by cytotoxic T cells or whether bona fide suppressor cells were involved. The SMC suppressed an MLR when the stimulator cells shared Bw57, Bw60, or Dw19 with the original stimulator. Separation of the SMC into CD4+ and CD8+ fractions demonstrated that the CD8+ fraction contained suppressive and cytotoxic activity against Bw57 or Bw60 antigens, while the CD4+ fraction contained both activities against the Dw19 specificity. The CD8+ fraction was also suppressive in the reverse MLR, while the CD4+ fraction was not. Limiting dilution analysis demonstrated that all CD8+-suppressive cultures were also cytotoxic, whereas in the CD4+ fraction both cytotoxic and non-cytotoxic suppressor cultures were found.

Specific suppression of mixed lymphocyte reactions by alloactivated cells is correlated with cytotoxicity

Mixed lymphocyte reaction (MLR)-activated lymphoblasts can suppress both proliferation and the induction of cell-mediated lysis (CML) when added to a subsequent MLR. Antigen specificity and the mechanism of MLR-induced suppression was investigated, with special emphasis on the exclusion of cell-mediated stimulator cell lysis as the cause of suppression. We studied the suppressive properties of nine MLR-activated cultures, selected because of their specific suppressive properties. Suppression of a given MLR was obtained when the stimulator cell carried at least one of the mismatched HLA antigens of the original stimulator. The mismatched antigen which activated suppression was an HLA class I antigen in 3 out of 9 cases, an HLA class II antigen in another 3 cases. In the remaining 3 cases suppression was observed when either an HLA class I or an HLA class II antigen was shared between the original stimulator and stimulator cell of the MLR. When analyzed on the same panel, a positive linear correlation between suppression of an MLR and CML activity against the stimulator cell was found. We also observed that both the original MLR (A*B) and the reserve MLR (B*A) could be suppressed by these anti-A suppressor cells. These results are compatible with the hypothesis that in the protocol studied suppression is mediated by lysis of the stimulator cells.

Regulatory mechanisms in cytotoxic T lymphocyte development. III. Induction, specificity, and genetic restriction of an in vitro suppressor T cell

We addressed questions pertaining to the immunogenetics of an in vitro alloinduced suppressor T cell (Ts) previously shown to inhibit cytotoxic T-lymphocyte (CTL) development by suppressing CTL precursor proliferation. Using intra-MHC recombinant strains of B10 congenic mice, the requirements for H-2 differences to induce Ts activity, the antigen specificity of the Ts, and the genetic restriction of Ts function were studied. It was found that differences at the K, D, or I regions alone can induce strong suppressor activity. Suppression of CTL development does not appear to be genetically restricted since the Ts inhibit CTL from responder cells disparate at K, K and D, I, or K and I. The alloinduced Ts is specific for the antigen stimulating its induction, but also inhibits CTL responses against immunologically unrelated determinants, even between class I and class II antigens, provided those determinants are carried on cells expressing the original inducing antigen. Ts can be triggered by antigens present on the responder cells but absent on the stimulator cells, indicating that the suppressive signal may be exerted directly on the responder population without specific interaction with stimulator cells.