High antigen concentration inhibits T cell proliferation but not interleukin 2 production: examination of limiting dilution microcultures and T cell clones

Cell cycle arrest caused by MEK/ERK signaling is a mechanism for suppressing growth of antigen-hyperstimulated effector T cells

Suppression of T-cell growth is an important mechanism for establishment of self-tolerance and prevention of unwanted prolonged immune responses that may cause tissue damage. Although negative selection of potentially self-reactive T cells in the thymus as well as in peripheral tissues has been extensively investigated and well documented, regulatory mechanisms to dampen proliferation of antigen-specific effector T cells in response to antigen stimulation remain largely unknown. Thus, in this work, we focus on the identification of growth suppression mechanisms of antigen-specific effector T cells. In order to address this issue, we investigated the cellular and molecular events in growth suppression of an ovalbumin (OVA)-specific T-cell clone after stimulation with a wide range of OVA-peptide concentrations. We observed that while an optimal dose of peptide leads to cell cycle progression and proliferation, higher doses of peptide reduced cell growth, a phenomenon that was previously termed high-dose suppression. Our analysis of this phenomenon indicated that high-dose suppression is a consequence of cell cycle arrest, but not Fas-Fas ligand-dependent apoptosis or T-cell anergy, and that this growth arrest occurs in S phase, accompanied by reduced expression of CDK2 and cyclin A. Importantly, inhibition of MEK/ERK activation eliminated this growth suppression and cell cycle arrest, while it reduced the proliferative response to optimal antigenic stimulation. These results suggest that cell cycle arrest is the major mechanism regulating antigen-specific effector T-cell expansion, and that the MEK/ERK signaling pathway has both positive and negative effects, depending on the strength of antigenic stimulation.

The [173–196] fragment of ovalbumin suppresses ovalbumin-specific rat IgE responses

Peptides and protein hydrolysates are attractive tools for the induction of tolerance or regulation of targeted B and/or T cell responses. In vivo, peptides are mainly produced by the action of digestive enzymes or following the processing of exogenous antigens by antigen-presenting cells (APCs). In vitro, these molecules are generally produced by enzymatic digestion and chemical hydrolysis of proteins. We investigated the T and B cell determinants of the major food allergen ovalbumin (nOVA) in rat by analyzing (1) the stimulatory effect of nOVA peptides generated by cyanogen bromide (CNBr) cleavage on nOVA-specific T cells, and (2) the potential of CNBr-derived OVA fractions to induce oral tolerance to nOVA. Peptide fractions of the CNBr-hydrolysated OVA were isolated by high-pressure liquid chromatography and tested for their ability to stimulate nOVA-specific T cells isolated from rats parenterally immunized with nOVA. The nOVA fractions containing the stimulatory determinants were then intragastrically administered to rat to test their potential to induce oral tolerance. The hole CNBr hydolysate stimulated proliferation of nOVA-specific T cells. Three out of the five HPLC-purified peptidic fractions were also able to stimulate proliferation and cytokine production by nOVA-specific T cells. A peptide fraction exhibiting a single peak by HPLC contained the 173-196 nOVA segment and stimulated nOVA-specific T cells. This segment also promoted oral tolerance to nOVA and reduced IgE responses. CNBr hydrolysis releases several peptides with stimulatory effect on nOVA-specific T cells including a new nOVA [173-196] T cell determinant which induces oral tolerance to nOVA.

Differential effect of CD8(+) and CD8(-) dendritic cells in the stimulation of secondary CD4(+) T cells

Dendritic cells (DC), in their role in initiation of the adaptive immune response, have been extensively studied for their capacity to interact and stimulate naive T cells. Subsets of mature murine DC isolated directly from the spleen have been shown to differ in their ability to induce proliferative responses in both primary CD4(+) and primary CD8(+) T cells; the myeloid-related CD8alpha(-) DC induce a more intense or prolonged proliferation of naive T cells than do the lymphoid-related DC bearing CD8alpha despite similar expression of MHC and co-stimulatory molecules. Here we examine the interaction of these DC subpopulations with T cells already in the activated or memory state which are known to have greater sensitivity to antigen stimulation and bear receptors with increased capacity for signal transduction. We show that influenza virus-specific CD4(+) T cell clones and splenic T cells from peptide-primed animals proliferated in response to antigen presented by separated splenic CD8(-) DC. In contrast, these T cells showed only weak, if any, proliferation in response to CD8(+) DC despite observable cluster formation in the cultures. The differential between the two DC types in inducing proliferation was even more pronounced than previously seen with primary T cells and did not reflect differential longevity of the DC in culture, altered response kinetics or deviation from IL-2 to IL-4 induction with CD8(+) DC, but was related to the levels of IL-2 induced. The deficiency in the CD8(+) DC was not overcome by using infectious virus rather than synthetic peptide as the antigen source. These results show that lymphoid-related CD8(+) splenic DC, despite their mature phenotype, fail to provide appropriate signals to secondary CD4(+) T cells to sustain their proliferation.

The role of CTLA-4 in tolerance induction and ttigen administration cell differentiation in experimental autoimmune encephalomyelitis: i. v. antigen administration

Interactions between B7 molecules on antigen-presenting cells and CTLA-4 on T cells have been shown to be important in establishing tolerance. In the present study, we examined the kinetics of tolerance induction following i.v. administration of myelin basic protein (MBP) Ac1-11 in mice transgenic for a TCR V(beta)8.2 gene derived from an encephalitogenic T cell clone specific for MBP Ac1-11. Examination of the lymph node cell (LNC) response 10 days after antigen administration demonstrated an accentuation of i.v. tolerance induction with anti-CTLA-4 blockade. Anergy was induced in splenocytes by i.v. antigen administration as shown by a decrease in MBP-specific proliferation and IL-2 production, and anti-CTLA-4 potentiated this effect. In addition, i.v. antigen plus anti-CTLA-4 and complete Freund's adjuvant was not encephalitogenic. Interestingly, i.v. tolerance (a single injection) did not inhibit experimental autoimmune encephalomyelitis (EAE) and anti-CTLA-4 administration did not alter this phenotype. These results suggest that while the majority of MBP-specific T cells are tolerized by i.v. antigen and that this process is potentiated by anti-CTLA-4 administration, a population of T cells remains that is quite efficient in mediating EAE.

Vaccination against Leishmania major in a CBA mouse model of infection: role of adjuvants and mechanism of protection

Gp63 is a major surface protein of Leishmania promastigotes. Its protective efficacy has been tested in several experimental models using different mouse strains, gp63 forms, adjuvants and routes of immunization, giving rise to conflicting results. This investigation was designed to determine whether these discrepancies could be ascribed to differing experimental procedures, and to compare gp63-induced protection with that achieved using live promastigotes. Preliminary experiments demonstrated that gp63 was an extremely potent immunogen compared to a standard antigen (ovalbumin). Protection against Leishmania major infection afforded by gp63 inoculation was studied in CBA mice. Injection of gp63 in saline, or of CFA, BCG, and C. parvum without antigen, induced significant protection. When gp63 and adjuvants were combined, results differed depending on the site of vaccination relative to that of the challenge infection. Vaccination with gp63 plus adjuvants in the tail (i.e. close to the site of infection) led to a stronger reduction of lesion size than the basal level of protection elicited by adjuvants alone, except in the case of CFA. Surprisingly however, when the antigen was injected at a distance from the site of infection (immunization in the hind foot pads, infection in the rump), the protective effect of gp63 was decreased by the adjuvants. Finally, vaccination at either site using live parasites (radioattenuated or virulent promastigotes) resulted in most instances in better protection than achieved by any protocol using gp63 and adjuvants. While anti-gp63 T cells proliferated in vitro in response to L. major-infected bone marrow-derived macrophages, they were unable to activate macrophages for parasite killing. This is in contrast with lymphocytes from mice immunized with live parasites, which both proliferated and stimulated significant killing of the microorganisms within 48 h.

Regulation of T-cell activation in the lung: alveolar macrophages induce reversible T-cell anergy in vitro associated with inhibition of interleukin-2 receptor signal transduction

Alveolar macrophages (AM) are recognized as archetypal 'activated' macrophages with respect to their capacity to suppress T-cell responses to antigen or mitogen, and this function has been ascribed an important role in the maintenance of local immunological homeostasis at the delicate blood:air interface. The present study demonstrates that this suppression involves a unique form of T-cell anergy, in which 'AM-suppressed' T cells proceed normally through virtually all phases of the activation sequence including Ca2+ flux, T-cell receptor (TCR) modulation, cytokine [including interleukin-2 (IL-2)] secretion and IL-2 receptor (IL-2R) expression. However, the 'suppressed' T cells fail to up-regulate CD2, and do not re-express normal levels of TCR-associated molecules after initial down-modulation; moreover, they are unable to transduce IL-2 signals leading to phosphorylation of IL-2R-associated proteins, and remained locked in G0/G1. The induction of this form of anergy is blocked by an NO-synthase inhibitor, and is reversible upon removal of AM from the T cells, which then proliferate in the absence of further stimulation. We hypothesize that this mechanism provides the means to limit the magnitude of local immune responses in this fragile tissue microenvironment, while preserving the capacity for generation of immunological memory against locally encountered antigens via clonal expansion of activated T cells after their subsequent migration to regional lymphoid organs. In an accompanying paper, we demonstrate that a significant proportion of T cells freshly isolated from lung exhibit a comparable surface phenotype.

Intravenous antigen administration as a therapy for autoimmune demyelinating disease

Experimental allergic encephalomyelitis is a prototypic autoimmune disease characterized by central nervous system inflammation and demyelination. Previously, we demonstrated that intravenous administration of high doses of myelin basic protein abrogated the clinical and pathological signs of experimental allergic encephalomyelitis by causing the deletion of encephalitogenic, CD4+, myelin basic protein-specific T cells through antigen-induced programmed cell death. In the present study, we further characterized the ability of intravenous antigen administration to attenuate an immune response by myelin basic protein-reactive encephalitogenic T cells. We demonstrated that multiple injections of myelin basic protein are required to achieve a therapeutic response, and that this form of therapy is effective even after prolonged chronic disease. These studies showed that although interleukin-2-stimulated cell cycling is an important factor leading to T-cell death, the administration of exogenous interleukin-2 with antigen can result in the aggravation of clinical disease compared to administration of antigen alone. More importantly, administration of myelin basic protein alone without interleukin-2 was sufficient to reduce autoreactive T cells and clinical disease in experimental autoimmune encephalomyelitis. Our experiments support the rationale for antigen-specific therapy aimed at inducing the programmed death of autoreactive T cells in autoimmune diseases, potentially including the human demyelinating disease multiple sclerosis.

T cell deletion in high antigen dose therapy of autoimmune encephalomyelitis

Encounters with antigen can stimulate T cells to become activated and proliferate, become nonresponsive to antigen, or to die. T cell death was shown to be a physiological response to interleukin-2-stimulated cell cycling and T cell receptor reengagement at high antigen doses. This feedback regulatory mechanism attenuates the immune response by deleting a portion of newly dividing, antigen-reactive T cells. This mechanism deleted autoreactive T cells and abrogated the clinical and pathological signs of autoimmune encephalomyelitis in mice after repetitive administration of myelin basic protein.

Function of dendritic cells and changes in T cell proliferation in antigen-induced nonresponsiveness

The ability of dendritic cells (DC) to acquire and present antigen to T cells during antigen-induced nonresponsiveness (AINR) in contact sensitivity was examined by studying cells from lymph nodes draining the sites of antigen challenge. Mice were pretreated on the right flank with either vehicle (AOO), oxazolone (Ox), or fluorescein isothiocyanate (FITC) and challenged 5, 10, or 20 days later with FITC on the left flank. At 5, 10, and 20 days, compared with animals pretreated with vehicle and challenged with FITC, those pretreated and challenged with FITC showed reduced acquisition of antigen by DC and the DC showed a reduced ability to stimulate naive T cells in vitro. Proliferation of T cells immediately on isolation (reflecting in vivo activity) was also reduced. When the time between pretreatment and challenge was extended to 40 days, the proliferative responses and antigen acquisition returned to normal. Animals sensitized with Ox and challenged with FITC showed nonspecific inhibition of T cell proliferation at 5 days only and not at later times and antigen levels on the DC from these animals were normal. The results show that low T cell proliferation during specific AINR in contact sensitivity may be a consequence of reduced acquisition and presentation of antigen by DC.

Analysis of phenotypic and functional changes during ganglioside-induced inhibition of human T cell proliferation

Glycosphingolipids added to the cell culture medium can be incorporated into the plasma membrane and interfere with the growth of certain cell types. In the past years, previous reports have shown that gangliosides, a class of glycosphingolipids bearing sialic acid can inhibit antigen or mitogen induced T cell proliferative responses in vitro. We report here that the inhibition of PHA induced proliferation by the trisialoganglioside GT1b was not reversed by addition of exogenous IL-1, IL-2, TPA and calcium ionophore. Furthermore, GT1b did not affect IL-2 production by activated T cells. In addition, GT1b ganglioside could also decrease strongly the expression of the T cell antigens CD3, CD2, CD4, CD8 and the alpha/beta T cell receptor antigenic complex whereas it did not affect HLA-class I antigens. By contrast, GT1b modulated only partially membrane expression of activation antigens such as CD25 (Tac) and transferrin receptor and increased the expression of HLA-class II antigens. Moreover CD25 messenger RNA induction was not affected by GT1b treatment of PHA-stimulated T cells. Our results demonstrate that gangliosides, in spite of their anti-proliferative capacity and their modulation effect on T cell antigen membrane expression, do not prevent the progression of T cells into early stages of the activation process.

Selective activation of Th1- and Th2-like cells in vivo--response to human collagen IV

Mice immunized with human collagen IV develop either antibody responses or T-cell proliferative responses as a function of the MHC genotype of the immunized mice. CD4+ T cells, similar to Th1 and Th2 cells, participate in these two types of responses, with CD4+ T-cell proliferative responses associating with IL-2 and IFN gamma release, and antibody production associating with CD4+ T-cell IL-4 and IL-5 release. Thus it would appear that the same antigen can induce responses consistent with either cell-mediated or humoral immunity depending on MHC class II genotype. In attempting to understand how MHC genotype controls the class of immunity observed several models are discussed. It was proposed, based on results obtained upon priming with human collagen IV, that the activation of Th1 and Th2 responses may be regulated at several levels (presentation by different APCs, presentation of different densities of the T-cell receptor ligand and presentation of different T-cell epitopes). With the identification of the peptide recognized by the CD4+ T cells, it was clear that the inability to induce Th1 responses in H-2b and the inability to induce Th2 responses in H-2s could not be accounted for by the failure to generate an immunodominant peptide during processing or the failure of the peptides to bind to the MHC class II molecules. Furthermore, the difference in the type of response generated could not be explained by the use of different peptides of the human collagen IV molecule in the two mouse strains, as a single peptide will induce both types of CD4+ T-cell response. However, it cannot be ruled out that the a2 peptide-class II interaction forms different T-cell ligands in the two strains either because the two class II MHC molecules are different or that the peptide is processed and reveals a different antigenic activity (Fox et al. 1988). Perhaps the most important finding from the peptide studies is that the lack of proliferative response in H-2b mice is not absolute, but can be overcome either by priming with high doses of the a2 peptide or by increasing the amount of peptide needed to elicit a recall response. It seems reasonable to speculate that changes in the dose required for priming Th1 or Th2 responses may reflect differences in the activation requirements of the two types of cells with Th1 cells requiring a high ligand density and Th2 cells a low ligand density.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro activation of immune lymph node cell proliferation by photohapten-modified cells in murine contact photosensitivity

Painting of 3,3',4',5-tetrachlorosalicylanilide (TCSA) plus ultraviolet A (UVA) irradiation to the same site induces contact photosensitivity (CPS), but at the same time results in death of the photohapten-modified cells. Using an in vitro immune lymph node cell (LNC) proliferation system, we investigated the mechanism of induction and elicitation of CPS by TCSA painting plus UVA irradiation. The proliferation of LNC from TCSA-photosensitized mice was not augmented by the addition of TCSA-photocoupled syngeneic spleen cells (SC) or epidermal cells (EC), whereas the picryl chloride immune LNC proliferation was activated by trinitrophenyl-coupled (TNP-coupled) SC or EC. While the viability of SC and EC was unchanged even after TNP haptenization, cells showed very low levels of viability after TCSA photohaptenization. This suggests that the inability of photoTCSA-modified cells to activate LNC proliferation is because of their low viability. Nylon wool column purified lymph node T cells from TCSA-photosensitized mice were activated by photohapten-conjugated SC or photohaptenized EC fragments only in the presence of peritoneal macrophages (M phi). The function of live M phi was not replaced by interleukin-1 (IL-1), suggesting that M phi were required for processing and/or presentation of photohapten rather than simply providing IL-1. Our in vitro study implies that photoTCSA-modified cells generated in vivo require intact antigen-presenting cells to effectively induce and elicit the CPS reaction.

Requirements for the growth of TH1 lymphocyte clones

Besides the signal generated in a T lymphocyte after triggering the T cell receptor (TcR), most lymphocytes need a "second signal" to become fully activated. The necessity and nature of the "second signal" differs between different types of T cells. At the level of CD4-positive T helper lymphocytes interleukin 1 (IL 1) serves as "second signal" for those of the TH2 subtype (IL4, 5, 6 producer) but not for those of the TH1 subtype (IL 2, IFN-gamma producer). This correlates with the absence of the IL 1 receptor at the surface of TH1 clones. We report herein the further purification of T cell stimulating factor (TSF), a soluble mediator involved in the proliferation of TH1 lymphocytes. A preparation free of detectable IL 1, 2, 4 and IL 6 activity could act as "second signal" required for the growth of TH1 lymphocytes in a TcR-mediated, as well as a TcR-independent activation system. In addition, we suggest that IL 1 can influence the proliferation of TH1 clones in an indirect way, probably via the induction of TSF in accessory cells.

A cell culture model for T lymphocyte clonal anergy

T lymphocytes respond to foreign antigens both by producing protein effector molecules known as lymphokines and by multiplying. Complete activation requires two signaling events, one through the antigen-specific receptor and one through the receptor for a costimulatory molecule. In the absence of the latter signal, the T cell makes only a partial response and, more importantly, enters an unresponsive state known as clonal anergy in which the T cell is incapable of producing its own growth hormone, interleukin-2, on restimulation. Our current understanding at the molecular level of this modulatory process and its relevance to T cell tolerance are reviewed.

B cells are required for induction of T cell abnormalities in a murine retrovirus-induced immunodeficiency syndrome

The role of B cells in induction of phenotypic and functional abnormalities of T cells in a murine retrovirus-induced immunodeficiency syndrome, MAIDS, was evaluated in mice depleted of mature B cells from birth with anti-IgM antibodies (mu-suppressed) and infected at 4 wk of age. Multicolor FACS analyses of CD4+ T cell subsets showed that development of phenotypic abnormalities of these cells at 9 wk after infection was completely inhibited by mu-suppression. Furthermore, induction of impaired proliferative responses to Con A and alloantigens and CTL responses to alloantigens was fully blocked in antibody-treated animals. The extent of virus replication was comparable in spleens of untreated and mu-suppressed mice. Retroviral induction of T cell dysfunction in MAIDS is thus dependent on the presence of B cells, and high level virus expression in mice without B cells has little or no effect on T cell function.

Two subsets of cloned T helper cells exhibit different activation requirements and characteristics

An anti-clonotypic monoclonal antibody (mAb 211G7H) was generated against a T helper cell clone which specifically recognized type II collagen. Besides inhibiting the proliferative response of the immunizing T cell clone to type II collagen, mAb 211G7H (soluble form) also suppressed the antigen-induced proliferation of several other T cell clones which shared similar specificity for antigen and major histocompatibility complex with the immunizing T cell clone. On the other hand, mAb 211G7H did not inhibit the responses of clones exhibiting different antigenic specificities from the clonotype-positive T cell clones. It was also demonstrated that these clonotype-positive T cell clones responded differently to mAb 211G7H when it was immobilized. Based upon their proliferative responses to immobilized anti-clonotype 211G7H mAb, two subpopulations of T cell clones were defined. Collagen-specific T cell clones of the first group proliferated poorly when stimulated with immobilized anti-clonotype mAb, by contrast, T cell clones belonging to the second group proliferated well to immobilized mAb. Furthermore, upon stimulation with immobilized 211G7H mAb, the second subpopulation of cloned T cells produced both interleukin (IL) 2 and IL4, while the first group secreted IL2 but not IL4. The cloned T cells from the group which responded weakly to immobilized anti-clonotype mAb also mediated reduced proliferative responses to IL2 in the presence of immobilized 211G7H mAb. Finally, these two subsets of T cell clones were found to respond differently to other non-antigen stimuli such as IL4 and phorbol 12-myristate 13-acetate. Thus, from a panel of collagen-specific T cell helper clones which had similar receptor fine specificities, we have isolated two subsets of cloned T helper cells that displayed different activation requirements and lymphokine production.

Do CD4 and CD8 control T-cell activation via a specific tyrosine protein kinase?

The CD4 and CD8 glycoproteins play an important role in T-cell activation by binding to major histocompatibility complex (MHC) class II or class I molecules, respectively, and stabilizing their interactions with the T-cell receptor-CD3 complex during antigen presentation. Recent evidence suggesting that the cytoplasmic domains of CD4 and CD8 are physically, and perhaps functionally, linked to the T-cell specific tyrosine protein kinase, p56lck, adds a new dimension to our current understanding of their physiological function. Based on these and other recent findings, Tomas Mustelin and Amnon Altman present a working hypothesis that defines a novel role for CD4 or CD8 in regulating T-cell activation, and perhaps other processes, such as thymic repertoire selection and human immunodeficiency virus (HIV)-induced immunosuppression.

Resistance to experimental autoimmune encephalomyelitis induced by neonatal tolerization to myelin basic protein: clonal elimination vs. regulation of autoaggressive lymphocytes

The target autoantigen of experimental autoimmune encephalomyelitis (EAE), myelin basic protein (MBP), appears late in ontogeny. In the rat MBP is expressed first on days 2-3 post partum, at a development stage, when self tolerance to most other autoantigens has already developed. To shed light on the cellular mechanisms that lead to immunological self tolerance to MBP, we treated neonatal rats with high doses of MBP before ontogenetic appearance of this autoantigen. We found that high doses are required to confer MBP-specific tolerance lasting until the adult life. Neonatally tolerized, adult rats are completely resistant to induction of EAE by injection of MBP in complete Freund's adjuvant (CFA). Upon MBP CFA challenge, these animals develop a limited humoral response to MBP, but are completely unreactive to MBP on the T cell level. The function of antigen-presenting cells is unchanged by neonatal tolerization, and there is no evidence for the induction of suppressive mechanisms. Transfers of large numbers of tolerized lymphocytes to normal hosts fails to interfere with EAE inducibility. Moreover, neonatally tolerized lymphocytes do not reduce MBP reactivity of primed lymph node cells or T line cells in vitro. Finally, neonatally tolerized rats are susceptible to EAE transferred by activated primed lymphocytes or by in vitro-activated MBP-specific T line cells. The apparent deletion of MBP-specific T lymphocytes in neonatally tolerized rats is in striking contrast to the physiological self tolerance to MBP, which is characterized by the presence of MBP-specific clones in the normal immune repertoire.

Antigen-presenting cells for unprimed T cells

The triggering requirements of T cells differ for primed and unprimed cells: primed T cells can be triggered to produce lymphokines without viable antigen-presenting cells (APCs), apparently by crosslinking the T-cell receptor (TCR). Unprimed T cells do, however, require viable APCs and here Jonathan Sprent and Mary Schaefer review what type of cells can carry out this function, with particular reference to APCs for unprimed CD8+ cells.

Viral-bacterial synergistic interaction in respiratory disease

In the present review we have identified how viruses can alter the host's susceptibility to bacterial infections by altering both environmental conditions in the lung which favor bacterial replication as well as by suppressing the host's defense mechanisms which prevent clearance of the bacteria. In many instances, these interactions are extremely complex but similar for many viruses. If the virus can overcome the initial host defense mechanisms, which include local antibody and mucus, the virus initiates tissue damage as a result of direct replication within the epithelial cells lining the mucosal surfaces of the respiratory tract. As a result of virus infection, the host cells respond by producing a variety of mediators including various types of interferons, which can alter both virus replication and host response. Replication also produces by-products of virus infection capable of initiating an inflammatory process, which in turn, through release of other mediators, can further modify lung defense mechanisms and encourage bacterial adherence and growth. The bacterium, in turn, releases chemotactic factors which encourage infiltration of specific effector cells into the lung. These effector cells can cause tissue damage and immunopathology, which encourage rapid bacterial growth and may result in death of the animal. In order to be able to control this complicated scenario, it is important either to prevent the initial infection with viruses or to reduce the degree of immunosuppression, so that bacterial clearance can occur rapidly before microcolony formation and extensive lung damage occur. Once a large amount of bacterial replication and lung damage is present, the use of antibiotics is generally of limited value. A schematic illustration of the complexity of the various interactions and counteractions occurring during virus--bacterial synergistic interactions is presented in Fig. 1.

Constitutive functional heterogeneity without detectable somatic mutation of antigen receptor genes in helper T cell clones: possible regulation by novel HLA class II "DY" determinants

The majority of interleukin 2-dependent HLA class II alloreactive human CD4-positive helper T-cell clones (TCC) presented the phenomenon of constitutive intraclonal heterogeneity, as reflected by limiting dilution analysis of changing frequencies of autonomously proliferative cells within the monoclonal population. Moreover, at 30-35 population doublings, these TCC lost their allospecific proliferative and helper capacity (phase I) and instead acquired strong antigen-non-specific suppressive activity (phase II). The TCR was still expressed at the same level on the cell surface. As shown by stable and identical rearrangement profiles of their T-cell receptor beta- and gamma-chain genes in both phases I and II, this constitutive change of function was probably not due to somatic mutation of the genes coding for the antigen-specific receptor. Moreover, antigen specific reactivity was retained in phase II TCC by the criterion of specifically stimulated secretion of granulocyte/macrophage colony stimulating factor. The preprogrammed intra-clonal functional flexibility is explained here by invoking a novel regulatory mechanism mediated by a fourth group of HLA class II determinants. When TCC were used as stimulators with the appropriate primed responders, it could be demonstrated that phase I autonomously proliferative non-suppressive T-helper lines failed to express certain novel lymphocyte activating determinants (LADs), whereas after their acquisition of suppressive function (phase II) these LADs were present. These stimulatory moieties appeared to represent a novel class II-like structure as established by serology, immunochemistry and functional characterization employing monoclonal antibodies to block stimulation. These operationally designated "DY" determinants are primarily and thus far exclusively involved in the induction of antigen non-specific suppressor cells. It is proposed that the intra-clonal changes of T-cell function described here are mediated by a switch in the utilization of qualitatively distinct class II-like restriction elements. The functional status of the phase I TCC can be altered by a switching over to the engagement of "DY" determinants in phase II. Thus, an autostimulatory and self-maintaining suppressive network may operate in extended inductive phases of human HLA restricted T-cell responses.

Inhibition and enhancement of in vitro helper activity of apocytochrome c-specific murine T cell populations and clones by anti-apocytochrome c-specific monoclonal antibodies

A murine monoclonal antibody (mAb) specific for apocytochrome c was found to be able to either inhibit or enhance the helper activity of mouse apocytochrome c-specific T cell clones and populations in a hapten (trinitrophenyl)-carrier (apocytochrome c) system of T-B cell cooperation. This effect of the mAb was carrier specific, could not be ascribed simply to a shift in the kinetics of the antibody response and was observed using apocytochrome c T helper cells of different mouse haplotypes. In addition, the anti-apocytochrome c mAb was able to inhibit specific T helper cell activity even when the T cells were triggered with antigen-presenting cells pulsed with antigen. Taken together, these results suggested that the mAb was inhibiting helper activity due to its ability to modify the interaction between T cells and antigen-presenting cells.