Alloreactive cytotoxic T cells. I. Alloreactive and allorestricted cytotoxic T cells

MHC restriction and allogeneic immune responses

Discovery of major histocompatability complex (MHC) restriction helped in the understanding of how T-lymphocytes recognize antigens on bacteria, viruses, and tumor cells. It was initially accepted that MHC restriction was a consequence of "adaptive differentiation" in the thymus; during differentiation, the forming repertoire of T-lymphocytes "learned" a low affinity for self MHC molecules via positive selection. This view was later countered by discovery of artifacts in underlying studies and the fact that adaptive differentiation could not explain direct allogeneic and allorestricted recognition phenomena. Data from experiments with TCR transgenic animals, individual MHC/peptide complex expression, and recipients of xenogenic thymus glands yielded evidence of an ability to adapt to microenvironment and a low specificity of positive selection. These facts led to an alternative interpretation of MHC restriction explained, in part, by specificity of a pool of effector cells activated by primary immunization. Details of this phenomenon were defined in studies that noted differential primary structures of peptides that bound various allelic forms of MHC molecules. Here, the T-lymphocyte repertoire formed in the thymus was a result, in part, of random rearrangement of germinal sequences of TCR gene fragments. Such pre-selected repertoires were inherently capable of reacting with different allelic forms of MHC molecules. In contrast, MHC molecules were characterized by significant intraspecies polymorphisms; negative and positive selections were aimed at adaptation of a pre-selected repertoire to a specific microenvironment in an individual. Via elimination of autoreactive clones and sparing of a broad spectrum of specificity to potential pathogens, selection in the thymus could be considered a life-long allogeneic reaction of a pre-selected repertoire to self MHC molecules resulting in tolerance to "self," increased responsiveness to foreign MHC molecules, and cross-reactivity of the mature T-lymphocyte repertoire to individual foreign peptides plus self MHC.

Stimulation with allogeneic Epstein-Barr virus-transformed lymphoblastoid cell lines generates HLA class I-specific CTLs with different target cell avidity

Epstein-Barr virus (EBV) transformed lymphoblastoid cell lines (LCL) are potent inducers of cytotoxic T-lymphocytes (CTL) in allogeneic mixed lymphocyte cultures (MLC). The contribution of EBV antigens to the induction of cytotoxic responses was investigated by comparing CTL clones derived from allogeneic MLCs of lymphocytes from one EBV seropositive and one seronegative donor for their capacity to lyse paired EBV positive and negative targets. The majority of the clones showed a conventional "HLA-specific" cytotoxicity and lysed equally well HLA-matched LCLs and mitogen-induced T- or B-blasts. A minority of the clones from both donors exhibited an "LCL-selective" killing potential as they lysed poorly T- and B-blasts. The LCL-selective clones did not recognize EBV antigens because they could not discriminate between EBV negative Burkitt lymphoma (BL) lines and their in vitro EBV-converted sublines. MAbs to CD3, CD8, and MHC class I antigens blocked the lysis of LCLs by HLA-specific and LCL-selective CTLs with comparable efficiency suggesting that the two effector types express T-cell receptors of similar affinity. T-blasts were unable to inhibit the lysis of LCLs in cross competition assays. This correlated with a significantly lower expression of the cell adhesion molecules ICAM-1 and LFA-3. The results suggest that stimulation with allogeneic LCLs activates HLA class I-specific CTLs with variable target cell avidity. Only CTLs that act independently of the enhancing effect of cell adhesion molecules are able to lyse mitogen-induced T- and B-blasts.

Manifestation of allo H-2-restriction specificity by self H-2-restricted T cells

Previous study has demonstrated that a considerable number of antigen-specific, allo H-2-restricted CTLPs could be recovered from normal lymphocytes. Experiments were conducted to examine whether antigen-specific, self H-2-restricted T cells could also manifest allo H-2-restriction specificity. Non-H-2-specific, self H-2-restricted CTLs were induced in the microcultures with a limited number of responder spleen cells from B10(H-2b) mice that had been primed with C3H.SW(H-2b) spleen cells. Two hundred thirteen non-H-2-specific CTLs generated in limiting dilution culture condition were assayed for cross-reactivity against TNP-modified and unmodified allogeneic targets of 10 different H-2 haplotypes. Of these CTLs, 29 (14%) were cross-reactive to a TNP-modified allogeneic target and seventeen (8%) to an unmodified allogeneic target. The quantitative analysis for cross-reactivity of non-H-2 specific, self H-2-restricted CTLs reveals a significant number of CTLs that were cross-reactive with either alloantigen or antigen (TNP) plus allo H-2. Furthermore, CTL clones with triple specificities including self H-2-restriction specificity, allo-reactivity, and allo H-2 restriction specificity were also found. These results indicate that antigen-specific, allo H-2-restricted T cells are distinct not only from alloreactive T cells but also from antigen-specific, self H-2-restricted T cells. Significance of the manifestation of allo H-2 restriction specificity by self H-2-restricted T cells is discussed in regard to the generation of T cell repertoire.

Growth-dependent variation of major histocompatibility complex (MHC) restriction and expression of Ly-2 and CD3/alpha/beta T cell receptor in cloned cytotoxic T cells

The specificity of major histocompatibility complex (MHC)-restricted antigen recognition by cytotoxic T cells (CTL) has been clearly correlated to the alpha/beta T cell receptor (TcR) complex on the T cell surface. Occasional changes in the specificity of in vitro cultivated CTL clones, therefore, have been suspected to result from alterations of the genes coding for the TcR alpha and/or beta chain. Here we demonstrate that pronounced variations in the stringency of MHC restriction, previously reported to occur during long-term culture of 2,4,6-trinitrophenyl (TNP)-specific CTL clones, may occur rapidly in a growth-dependent, reversible manner, i.e. without structural TcR variation. Several H-2b TNP-specific CTL clones were shown to possess strong cross-reactivity for H-2k TNP target cells when seeded at low cell numbers, but exhibit reduced or undetectable cross-reaction to H-2k TNP in high-density cultures. Another clone revealed "heteroclitic" properties with significantly stronger cytotoxic activity towards allogeneic (H-2k) than syngeneic (H-2b) TNP-modified target cells. In this case dilute cultures appeared as exclusively allo-MHC restricted, whereas dense cultures were allo/self cross-restricted. In all instances these phenomena were accompanied by cell density-dependent quantitative changes in the expression of Ly-2 and T cell antigen receptor. CTL from dilute cultures had at least 2-fold higher surface concentrations of Ly-2 and CD3 antigens than cells from dense cultures while other surface markers such as Thy-1 or LFA-1 were completely identical. No such effects were observed for CTL clones exhibiting cell density-independent specificity patterns. We conclude from these findings that (a) the stringency of MHC restriction specificity may be significantly affected by the amount of expressed TcR and/or Ly-2 molecules, (b) CTL possess mechanisms to regulate Ly-2 and TcR expression and, hence, their MHC-restricted antigen recognition, and (c) the ability to regulate Ly-2 and TcR expression may be altered during prolonged culture of a CTL clone.

Clonal specificity analysis of mitogen-activated murine T lymphoblasts

We have determined the frequencies and specificities of MHC-reactive and MHC-restricted cytotoxic T lymphocyte precursors (CTL-p) in mitogen (ConA)-activated splenocytes of normal unprimed mice. The limiting dilution (LD) system supported the growth of one out of three Lyt2+ T cell blasts. The generated CTL-populations lysed blast cell targets specifically as determined by split well analyses. MHC-gene product expression was necessary for lysis to occur, since MHC-negative F9 teratocarcinoma cells were not lysed. The frequency determinations and split well analyses revealed: 1) equally high numbers (approximately 1/100) of CTL-p that generated specific allo-MHC or self-MHC reactive CTL populations, 2) high frequencies of CTL-p which recognized hapten (TNP) or minor H (MH)-antigens in the context of self MHC or allo-MHC determinants. The results are discussed with respect to antigen, restriction and receptor specificities of mitogen-activated unprimed T cell blasts.

Specificity repertoire of splenic Lyt-2+/F23+ cytotoxic lymphocyte precursors from B6 mice

As revealed by flow cytometric analysis, about 30% of nylon wool nonadherent Lyt-2+ B6 spleen cells were F23+, i.e., were stained with the monoclonal antibody F23.1 directed against an allotypic T-cell receptor determinant. The specificity repertoire of splenic Lyt-2+/F23+ cytotoxic lymphocyte precursors (CLP) from B6 mice was investigated in a limiting dilution (LD) system designed to support clonal expansion in vitro of a representative fraction of this T-cell subset: in highly purified Lyt-2+ responder cells cocultured with mitomycin-treated F23 hybridoma cells in the presence of (recombinant) interleukin 2 under LD conditions, one out of three Lyt-2+/F23+ CLP gave rise to a functional cytotoxic T lymphocyte (CTL) clone. The split-well analysis of individual CTL populations demonstrated a clear-cut segregation of the lytic reactivities toward different allogeneic Con A blast targets. A large fraction of B6-derived CTL clones (3-10%) specifically lysed fully H-2 allogeneic (H-2k, H-2d), or H-2K mutant (bm1) targets. Self-reactive and allorestricted lytic patterns were not found.

A novel cytotoxic T-cell clone that exhibits differential modes of recognition in the proliferative and cytolytic phase

A T-cell clone (1G8-H7) cytotoxic to P815Y mastocytoma (H-2d) has been established from spleen cells of a C3H/He mouse (H-2k) primed with P815Y cells by means of in vitro stimulation with irradiated C3H.H-2o(H-2KdDk) spleen cells. The clone 1G8-H7 was an interleukin 2 (IL-2)-dependent and H-2Kd antigen-dependent CTL clone and it killed P815Y cells but not Concanavalin A-induced spleen blast cells bearing H-2Kd antigen. The involvement of H-2Kd antigen in the cytolytic recognition mechanism was shown by the inhibition of lysis by anti-H-2Kd monoclonal antibody and also by the cold inhibition experiment that employed H-2Kd-bearing spleen cells. Comparison of cytotoxic activities between 1G8-H7 and Kd-specific CTL clones showed that the killing of P815Y cells by clone 1G8-H7 was not explained by the susceptibility to cell-mediated cytolysis of P815Y cells. These results suggest that H-2Kd antigen on the stimulating cell is sufficient to deliver a proliferation signal in the proliferative phase of this clone, but in the cytolytic phase an additional interaction with surface structure on the target cell other than that with H-2Kd antigen is required for the induction of cytolysis. Possible elucidations for the differential modes of recognition are discussed.

Use of V beta.8 genes in splenic Lyt-2+ cytotoxic lymphocyte precursors reactive to bm1 or bm14 alloantigen in individual C57BL/6 mice

The cytotoxic response of cell sorter-purified small Lyt-2+ splenic cytotoxic lymphocyte precursors from 10 individual C57BL/6 mice to mutant class I H-2Kbm1 or H-2Dbm14 allodeterminants was analyzed under limiting dilution conditions. The cytotoxic activity of anti-bm1-specific or anti-bm14-specific cytotoxic T lymphocyte (CTL) populations (selected for a high probability of clonality) was tested against F23 hybridoma cells; F23+ CTL clones lysed F23 hybridoma targets but F23- CTL clones did not. In the C57BL/6 anti-bm1 mixed lymphocyte reaction, 36% (range 29-48%) of the generated CTL clones were F23+; in the B6-anti-bm14 mixed lymphocyte reaction, 45% (range 34-49%) of the generated CTL clones were F23+. Hence, a large fraction of the anti-bm1- or anti-bm14-reactive CTL clones from C57BL/6 mice use V beta.8 genes to construct these allospecific T cell receptor phenotypes, but no extensive variation in the use of V beta.8 genes in the construction of allospecific T cell receptor phenotypes of restricted heterogeneity is found in individual mice of the same strain.

Significant frequency of cytotoxic T lymphocyte precursor cells specific for TNP-modified allogeneic cells in normal lymphocytes

It was tested whether the cytotoxic T-lymphocyte precursor (CLP) repertoire in normal mice is biased toward recognizing foreign antigen in association with self H-2 as opposed to allogeneic H-2. The frequencies of CLPs in normal mice (H-2b,k,d) specific for TNP-modified syngeneic and TNP-modified allogeneic cells have been compared by limiting dilution analysis. Normal spleen cells were cultured at a limiting dilution with TNP-modified (TNP-self) or TNP-modified allogeneic (TNP-allo) stimulator cells. Cultures were split into four aliquots and assayed against TNP-self, TNP-allo, unmodified syngeneic, and unmodified allogeneic Concanavalin A blast targets and classified for cytotoxic activity directed against TNP-self, TNP-allo, and allo H-2 determinants. In disagreement with our expectations from the literature, the frequencies of CLPs in H-2b and H-2d responder cells recognizing TNP-modified H-2k were higher than the frequencies of CLPs recognizing TNP-self. There was no clear preference for TNP-self in the case of H-2b responder and H-2d allogeneic cells, nor vice versa. Only in the case of H-2k responder cells was there a distinct preference for TNP-self. The significance of a considerable number of TNP-specific, allo H-2-restricted CLPs in normal lymphocytes is discussed.

Detection of rearranged T cell receptor beta-chain gene and induction of cytolytic function in interleukin 2-responsive day 14-15 murine fetal thymocytes

A subpopulation of interleukin 2 (IL 2) receptor-positive day 14-15 murine fetal thymocytes can be induced by recombinant IL 2 to proliferate over prolonged time periods in dissociated cell cultures. The proliferating day 14-15 fetal thymocytes exhibit no cytolytic effector function, nor do they rearrange T cell receptor beta chain genes. This contrasts with thymic organ cultures in which day 14-15 thymocytes do rearrange beta chain genes and give rise to immunocompetent cells. However, such events can also take place in dissociated cell cultures, provided the IL 2-responsive thymocytes are cultured on syngeneic feeder cells in the presence of IL 2 and the mitogen concanavalin A. Under such conditions rearrangement of the beta chain gene complex becomes detectable and cytolytic effector cells are generated. The frequency of inducible cytolytic precursor cells in day 14-15 thymocytes is 1/7000. These data either imply that immunocompetent cells are already present in the day 14-15 fetal thymus, or differentiation from precursors to immunocompetent cells must occur in dissociated cell cultures.

Hapten-specific cytotoxic T cell clones undergo somatic variation of their antigen recognition specificity

Two experimental systems have demonstrated somatic variation of antigen recognition specificity of long-term cytotoxic T cell (CTL) clones. System 1 used CTL clone BT7.4.1 with strict specificity for Kb/TNP, which had been continuously cultured for 15 months in the presence of H-2b/TNP stimulator cells and interleukin 2. Upon removal of the TNP antigen from the cultures, 99% of the clone cells within about 10 cell divisions lost their ability to grow in the presence of antigen and interleukin 2 (lethal variants). Of the surviving 1%, about 60% retained the ability to lyse target cells in the presence of lectins but only 12% could be considered as "wild type" BT7.4.1 cells, i.e. they still specifically lysed H-2b/TNP-bearing target cells. The majority of the growing cells, thus, had to be considered as specificity loss variants. Several specificity loss variants were established in culture and were shown to express membrane-bound T cell "receptor" heterodimer similar to their TNP-specific ancestor, BT7.4.1. Principally the same types of variants were generated in cultures growing in the presence of TNP antigen, although in quantitatively reduced numbers. Under these conditions the specific stimulator cells appeared to impose a significant selective advantage for "wild type" CTL since even after 15 months the cultures fully retained their specificity for the nominal antigen. In system 2, the development of cytolytic fine specificity of a panel of 42 individual Kb/TNP-specific CTL clones was followed over a period of 8 months of in vitro culture. At the beginning of the test, 37 of these clones exhibited significant cross-reactivity for lysis of H-2k/TNP target cells. This number of cross-reactive clones continuously diminished with time and dropped to only 4 clones after 8 months in culture. All 42 clones retained their original Kb/TNP specificity and after losing their reactivity for H-2k/TNP usually showed no decrease but rather an increase in their cytotoxic activity towards Kb/TNP target cells. Loss of H-2k/TNP cross-reactivity was not accompanied by loss of Lyt-2 or of LFA-1 surface antigens or by loss of sensitivity of the cytotoxicity to inhibition by anti-Lyt-2 or by anti-LFA-1 antibody. We conclude from these observations that in vitro cultivated CTL clones, at least those of C57BL/6 anti-TNP-C57BL/6 specificity, are not stable in terms of their antigen recognition specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

Allorestricted cytotoxic T cells. Large numbers of allo-H-2Kb-restricted antihapten and antiviral cytotoxic T cell populations clonally develop in vitro from murine splenic precursor T cells

Cytotoxic T lymphocyte (CTL) responses of splenic T cells from C57BL/6 B6) mice and mutant H-2Kbm1 (bm1) mice to haptenic (trinitrophenyl [TNP] ) and herpes simplex virus (HSV) determinants in the context of an allogenic (wild-type or mutant) H-2Kb molecule were analyzed in a modified limiting dilution system. In the B6-anti-bm1TNP mixed leukocyte reaction (MLR), estimated frequencies for precursors of CTL clones that lysed bm1TNP targets ranged from 1/120 to 1/400; in the bm1-anti-B6TNP MLR, estimated frequencies of precursors of CTL clones that lysed B6TNP targets ranged from 1/500 to 1/1,300. Estimated frequencies for precursors of CTL clones that lysed the respective unmodified and TNP-modified allogeneic targets were two- to three-fold lower. Lytic specificity patterns determined by split-well analysis showed that at least 20-30% of the generated CTL populations (selected for a high probability of clonality) in both MLR displayed allorestricted lysis of TNP-modified concanavalin A blast targets. In the B6-anti-bm1HSV MLR, estimated frequencies for precursors of CTL clones that lysed bm1HSV targets ranged from 1/70 to 1/300; in the bm1-anti-B6HSV MLR, estimated frequencies for precursors of CTL clones that lysed B6HSV targets ranged from 1/300 to 1/1,200. Again, estimated frequencies for precursors of CTL clones that lysed the respective noninfected and virus-infected allogeneic targets were two- to fourfold lower. Of the CTL populations selected for a high probability of clonality at least 30-60% displayed allorestricted lysis of virus-infected lipopolysaccharide blast targets in both MLR. It is concluded that a large fraction of clonally developing CTL populations stimulated with TNP-modified or HSV-infected allo-H-2Kb-bearing cells displayed an allorestricted pattern of recognition. It was further evident that the estimated frequencies of splenic precursors that generated allorestricted CTL clones was two- to threefold higher than the estimated frequencies of precursors that gave rise to the respective alloreactive CTL populations.