Early impairment of hepatitis C virus specific T cell proliferation during acute infection leads to failure of viral clearance

BACKGROUND AND AIMS

Cellular mediated immunity (CMI) is thought to play a key role in resolution of primary hepatitis C virus (HCV) infection. However, CD4+ and CD8+ T cell responses are also generated during acute infection in individuals who become chronic, suggesting that they developed a defective CMI. The aim of this study was to verify if and when such immune dysfunction is established by measuring the breadth, magnitude, function, and duration of CMI in a large cohort of subjects during the natural course of acute HCV infection.

METHODS

CMI was comprehensively studied by prospective sampling of 31 HCV acutely infected subjects enrolled at the onset of infection and followed for a median period of one year.

RESULTS

Our results indicated that while at the onset of acute HCV infection a measurable CMI with effector function was detected in the majority of subjects, after approximately six months less than 10% of chronically infected individuals displayed significant CMI compared with 70% of subjects who cleared the virus. We showed that progressive disappearance of HCV specific T cells from the peripheral blood of chronic patients was due to an impaired ability to proliferate that could be rescued in vitro by concomitant exposure to interleukin 2 and the antigen.

CONCLUSION

Our data provide evidence of strong and multispecific T cell responses with a sustained ability to proliferate in response to antigen stimulation as reliable pharmacodynamic measures of a protective CMI during acute infection, and suggest that early impairment of proliferation may contribute to loss of T cell response and chronic HCV persistence.

Multispecific T cell response and negative HCV RNA tests during acute HCV infection are early prognostic factors of spontaneous clearance

BACKGROUND/AIMS

Hepatitis C virus (HCV) infection results in a high frequency of chronic disease. The aim of this study was to identify early prognostic markers of disease resolution by performing a comprehensive analysis of viral and host factors during the natural course of acute HCV infection.

METHODS

The clinical course of acute hepatitis C was determined in 34 consecutive patients. Epidemiological and virological parameters, as well as cell mediated immunity (CMI) and distribution of human leukocyte antigens (HLA) alleles were analysed.

RESULTS

Ten out of 34 patients experienced self-limiting infection, with most resolving patients showing fast kinetics of viral clearance: at least one negative HCV RNA test during this phase predicted a favourable outcome. Among other clinical epidemiological parameters measured, the self-limiting course was significantly associated with higher median peak bilirubin levels at the onset of disease, and with the female sex, but only the latter parameter was independently associated after multivariate analysis. No significant differences between self-limiting or chronic course were observed for the distribution of DRB1 and DQB1 alleles. HCV specific T cell response was more frequently detected during acute HCV infection, than in patients with chronic HCV disease. A significantly broader T cell response was found in patients with self-limiting infection than in those with chronic evolving acute hepatitis C.

CONCLUSION

The results suggest that host related factors, in particular sex and CMI, play a crucial role in the spontaneous clearance of this virus. Most importantly, a negative HCV RNA test and broad CMI within the first month after onset of the symptoms represent very efficacious predictors of viral clearance and could thus be used as criteria in selecting candidates for early antiviral treatment.

Both maturation and survival of human dendritic cells are impaired in the presence of anergic/suppressor T cells

T cell suppression is a well established phenomenon, but the mechanisms involved are still a matter of debate. Mouse anergic T cells were shown to suppress responder T cell activation by inhibiting the antigen presenting function of DC. In the present work we studied the effects of co-culturing human anergic CD4⁺ T cells with autologous dendritic cells (DC) at different stages of maturation. Either DC maturation or survival, depending on whether immature or mature DC where used as APC, was impaired in the presence of anergic cells. Indeed, MHC and costimulatory molecule up-regulation was inhibited in immature DC, whereas apoptotic phenomena were favored in mature DC and consequently in responder T cells. Defective ligation of CD40 by CD40L (CD154) was responsible for CD95-mediated and spontaneous apoptosis of DC as well as for a failure of their maturation process. These findings indicate that lack of activation of CD40 on DC by CD40L-defective anergic cells might be the primary event involved in T cell suppression and support the role of CD40 signaling in regulating both activation and survival of DC.

Mitogen-activated kinase kinase kinase 1 regulates T cell receptor- and CD28-mediated signaling events which lead to NF-kappaB activation

Optimal activation of Rel/NF-kappaB transcription factors in T lymphocytes requires a CD28-delivered co-stimulatory signal in addition to TCR engagement. Although, Rel/NF-kappaB transcription factors are critical regulators of many T cell functions, the mechanisms and molecules, which link the surface receptors to their activation, are poorly characterized. Using Jurkat T cells stimulated with superantigen presented on B7-positive APC, we showed that CD28- and TCR-stimulated NF-kappaB-dependent transcription is associated to the activation of IkappaB kinase beta (IKKbeta) and, to a lesser extent, of IkappaB kinase alpha (IKKalpha). A dominant negative mutant of the MAP3 kinase MEKK1, a kinase known to regulate the JNK pathway and to activate NF-kappaB-dependent transcription in many cell types, strongly inhibits CD28- and TCR-induced IKK activity, whereas the dominant negative mutants of the NF-kappaB-inducing kinase (NIK) did not exert any significant effects. In addition, TCR/CD28 stimulation results in the recruitment and autophosphorylation of endogenous MEKK1, whereas endogenous NIK was not detectably activated. Our data identify MEKK1 as a critical step in coupling signals initiated by TCR and CD28 to the downstream pathways which lead to both AP-1 and NF-kappaB activation in T lymphocytes.

Peptide analogues as a strategy to induce tolerance in T cells with indirect allospecificity

BACKGROUND

It has been demonstrated that indirect recognition of allogeneic MHC molecules might play an important role in provoking graft rejection. Although direct recognition of allogeneic molecules on antigen presenting cells of the graft may induce a state of tolerance, the continuous presentation of processed alloantigens by specialized antigen presenting cells does not allow the same phenomenon to occur. Tolerance to interleukin-2 secreting T cells can be achieved in different ways, among these is the exposure to mutants of the wild type allopeptide. We have investigated whether peptide analogues of the allopeptide can induce tolerance in T cells with indirect allospecificity.

METHODS

T cell clones with indirect anti-HLA-A2-specificity generated from a HLA-A2-DRB1*1502+ patient who chronically rejected a HLA-A2-expressing kidney allograft were used for this study. Nine peptide analogues of HLA-A2 (residues: 103-120) were produced with single amino acid substitutions at the putative T cell receptor for antigen contact positions. Their effect on the proliferation of a panel of T cell clones was evaluated.

RESULTS

Peptide analogues and wild type peptide had similar capacity to bind to the restriction molecule HLA-DRB1*1502. Co-presentation of the peptide analogues 111R/A, H, K and 114H/K, with the wild type peptide inhibited T cell responses, indicative of antagonism. In addition, one analogue 112G/S induced unresponsiveness in the T cells to subsequent culture with the wild type peptide.

CONCLUSIONS

The data presented here suggest that using reagents such as altered peptides may represent a strategy to prevent the activation of T cells with indirect alloreactivity and allograft rejection in vivo.

Engagement of CD4 before TCR triggering regulates both Bax- and Fas (CD95)-mediated apoptosis

In the present study, we have aimed at clarifying the CD4-dependent molecular mechanisms that regulate human memory T cell susceptibility to both Fas (CD95)-dependent and Bcl-2-dependent apoptotic pathways following antigenic challenge. To address this issue, we used an experimental system of viral and alloantigen-specific T cell lines and clones and two ligands of CD4 molecules, Leu-3a mAb and HIV gp120. We demonstrate that CD4 engagement before TCR triggering suppresses the TCR-mediated neosynthesis of the Flice-like inhibitory protein and transforms memory T cells from a CD95-resistant to a CD95-susceptible phenotype. Moreover, evidence that the apoptotic programs were executed while Fas ligand mRNA expression was inhibited led us to analyze Bcl-2-dependent pathways. The data show that the engagement of CD4 separately from TCR influences the expression of the proapoptotic protein Bax independently of the anti-apoptotic protein Bcl-2, whereas Ag activation coordinately modulates both Bax and Bcl-2. The increased expression of Bax and the consequent dissipation of the mitochondrial transmembrane potential (DeltaPsim) suggest a novel immunoregulatory function of CD4 and demonstrate that both passive cell death and activation-induced cell death are operative in CD4+ memory T cells. Furthermore, analysis of the mechanisms by which IL-2 and IL-4 cytokines exert their protective function on CD4+ T cells in the presence of soluble CD4 ligands shows that they were able to revert susceptibility to Bax-mediated but not to CD95-dependent apoptotic pathways.

High prevalence of hypervariable region 1-specific and -cross-reactive CD4(+) T cells in HCV-infected individuals responsive to IFN-alpha treatment

The hypervariable region 1 (HVR1) of the putative envelope 2 protein of the hepatitis C virus (HCV) is the most variable part of the whole HCV polyprotein. Anti-HVR1 antibodies have been shown to protect against HCV infection, indicating that this region contains an important neutralization determinant. Recently we and others have demonstrated that HVR1 is also a T cell determinant able to activate helper T cell responses during HCV infection. In order to investigate the role of the immune response against HVR1 during HCV infection we have evaluated the humoral and lymphoproliferative responses to a panel of HVR1 peptides in HCV-infected patients with different outcomes of the disease following interferon-alpha (IFN-alpha) treatment. We observed that the frequency of anti-HVR1 T cell responses was significantly higher in patients who recovered after IFN-alpha therapy than in those who did not, while no differences in the anti-HVR1 antibody reactivities were detected. In addition, by generating HVR1-specific T cell lines and clones we identified human leukocyte-associated antigens DR4 restricted T cell epitopes in the carboxy-terminus of HVR1 and we demonstrated that broadly cross-reactive HVR1 T cells are elicited by HVR1.

Use of a human minichromosome as a cloning and expression vector for mammalian cells

A natural human minichromosome (MC1) derived from human chromosome 1 was shown to be linear and to have a size of 5.5 Mb. Human IL-2 cDNA and the neo gene were co-transfected into a MC1-containing human-CHO hybrid cell line. Integration of the foreign genes was directed to the pericentromeric region of MC1 by co-transfection of chromosome 1-specific satellite 2 DNA. A number of G418-resistant transfectants were obtained and expression of IL-2 was determined. FISH analysis demonstrated co-localization in the minichromosome of the IL-2 gene and of the satellite 2 DNA. An IL-2-producing clone was used in cell fusion experiments with IL-2-dependent murine CTLL cells to generate CTLL-human hybrids containing the modified minichromosome (MC1- IL2 ). The hybrids were able to grow in medium lacking IL-2 for 17 mean population doublings (MPD), indicating that expression of the cytokine was sufficient to relieve the IL-2 dependence of CTLL proliferation. Endogenous IL-2 production delayed the onset of apoptosis in the IL-2-dependent CTLL cells. Mitotic stability was shown to be 100% in the human-CHO hybrids and 97% per MPD in CTLL cells. These results demonstrate that a natural human minichromosome can be utilized as a cloning and expression vector for mammalian cells and that the MC1 minichromosome can be engineered to deliver IL-2 to two types of cells, fibroblasts and lymphocytes.

Hypervariable region 1 variants act as TCR antagonists for hepatitis C virus-specific CD4+ T cells

In various human viral infections, the appearance of mutated epitopes displaying TCR antagonistic activity has been correlated with the severity and persistence of infection. In hepatitis C virus (HCV) infection, where the virus persistence has been associated with the rapid and substantial Ag modifications occurring during replication, TCR antagonism has been evidenced in CD8+ T cell responses. However, CD4+ T cell antagonism may be another important strategy by which HCV eludes a protective response, because sustained Th responses directed against several HCV Ags are associated with a self-limited course of infection. The data reported here represent the first evidence that variants of the hypervariable region (HVR1) of the putative Envelope 2 protein of HCV can act as powerful TCR antagonists for HVR1-specific CD4+ T cells isolated from HCV-infected individuals. Using classical antagonism assays, we observed strong inhibition of cellular proliferation and cytokine production when the agonist and the antagonist ligands were simultaneously presented by the same APCs. The presence in HVR1 of conserved residues, critical for binding to HLA-DR molecules, supports the function of HVR1 variants as TCR antagonists. In conclusion, our data evidence an antagonism phenomenon, which was achieved by naturally occurring class II-restricted T cell epitopes whose mechanism was addressed in terms of the antagonist capacity to inhibit agonist-mediated TCR down-regulation and early signal transduction.

TCR engagement regulates differential responsiveness of human memory T cells to Fas (CD95)-mediated apoptosis

In this work, we have tried to establish whether human memory T cells may be protected from Fas (CD95)-induced apoptosis when correctly activated by Ag, and not protected when nonspecifically or incorrectly activated. In particular, we wanted to investigate the molecular mechanisms that regulate the fate of memory T cells following an antigenic challenge. To address this issue, we chose an experimental system that closely mimics physiological T cell activation such as human T cell lines and clones specific for viral peptides or alloantigens. We demonstrate that memory T cells acquire an activation-induced cell death (AICD)-resistant phenotype when TCRs are properly engaged by specific Ag bound to MHC molecules. Ag concentration and costimulation are critical parameters in regulating the protective effect. The analysis of the mechanisms involved in the block of CD95 signal transduction pathways revealed that the crucial events are the inhibition of CD95-associated IL-1beta-converting enzyme (ICE)-like protease (FLICE) activation and poly(ADP)-ribose polymerase cleavage, and the mRNA expression of FLICE-like inhibitory protein. Furthermore, we have observed that TCR-mediated neosynthesis of FLICE-like inhibitory protein mRNA is suppressed either by protein tyrosine kinase inhibitors or cyclosporin A. In conclusion, the present analysis of the effects of TCR triggering on the regulation of AICD suggests that AICD could be inhibited in human memory T cells activated in vivo by a foreign Ag, but may become operative when the Ag has been cleared.

Dexamethasone induces apoptosis in human T cell clones expressing low levels of Bcl-2

Previous results of ours have demonstrated that the same clonotype can express both a sensitive and a resistant phenotype to Dex-mediated PCD induction depending on its cell cycle phase. In particular, we demonstrated that human T lymphocytes, arrested in the G0/G1 phase of the cell cycle, are susceptible, while proliferating T cells are resistant to Dex-mediated apoptosis. In this paper, we have further characterized the sensitive and resistant phenotypes and investigated whether a different expression of the apoptotic genes Fas, FasL, Bcl-2, Bcl-x and Bax is involved in the regulation of Dex-mediated apoptosis. The results show that the amount of Bcl-2 expression, that changes during cell cycle phases, determines susceptibility or resistance to apoptosis induced by Dex. In fact, undetectable expression of Bcl-2 in sensitive cells favors Dex-mediated apoptosis while high expression of Bcl-2 in proliferating cells counterbalances apoptosis induction. Moreover, the addition of exogenous IL-2, in the presence of Dex, fails to up-regulate Bcl-2 expression and to revert Dex-mediated apoptotic phenomena.

CD4+ T cells orchestrate both amplification and deletion of CD8+ T cells

This review focuses on the role of CD4+ T cells in regulating immune responses, orchestrating both the amplification and deletion of immune cells, particularly CD8+ T cells. These two functions, which represent only an apparent contradiction, appear to be two faces of the same process of regulation. In fact, because the immune response, once activated, needs to be carefully controlled or switched off when the antigenic stimulus is eliminated, the immune system has developed several strategies either to regulate clonal amplification or to avoid useless expansion of activated cells. In particular, we have reported many data demonstrating that CD4+ T cells may be indicated as the regulatory element in the activation as well as the deletion of CD8+ T cells. New data are also reported on the ability of anergic CD4+ T cells to suppress CD8+ T-cell activation through induction of apoptosis, and on the need for CD8+ T cells for antigen recognition in inducing cell death in CD4+ T cells. Moreover, the central role of CD4+ T cells in the maintenance of peripheral tolerance has been widely described.

Effect of caffeic acid on tert-butyl hydroperoxide-induced oxidative stress in U937

Nonvitamin phenolic compounds are ubiquitous in food plants and therefore potentially present in human plasma in a diet-dependent concentration. The aim of this study was to evaluate the ability of caffeic acid, a phenolic acid with antioxidant activity, to affect cellular response in U937 human monocytic cells to t-butyl hydroperoxide-induced oxidative stress. In our experimental conditions caffeic acid was incorporated into cells without any cytotoxic effect. Caffeic acid-treated cells showed an increased resistance to oxidative challenge, as revealed by an higher percent of survival and the maintenance of an higher proliferative capacity in respect to control cells. This effect seems to be due to the ability of caffeic acid to reduce glutathione depletion and to inhibit lipid peroxidation during tBOOH treatment. It can be concluded that caffeic acid exerts an antioxidant action inside the cell, responsible for the observed modulation of the cellular response to oxidative challenge. Due to its presence in the diet, therefore, caffeic acid may play a role in the modulation of oxidative processes in vivo.

Dissociation of T cell anergy from apoptosis by blockade of Fas/Apo-1 (CD95) signaling

Induction of anergy and deletion due to apoptosis are two of the mechanisms involved in peripheral tolerance. To clarify the relationship between these two phenomena we have used an in vitro system of T cell Ag presentation. The recognition of Ag displayed by MHC class II-expressing T cells (T-APC) induces partial signals in Ag-specific T cell clones. This leads to a blunted intracellular calcium flux, and the T cells become unable to proliferate in response to further challenge with professional APC. These T cells are unable to produce IL-2, but retain the ability to release IL-4. In the present study, we report that for some T cell clones, the predominant outcome of Ag recognition on T cells is cell death. For susceptible T cell clones, the number of cells that die is proportional to the peptide concentration. This cell death resulted from Fas/Apo-1 (CD95)/Fas-ligand interactions between the T cells, in that Fas ligand expression was detected following overnight culture of T cells with T-APC and neutralizing anti-CD95 Ab protected from death. Most notably, following anti-CD95-mediated protection from apoptosis, the rescued T cells remained unable to respond to rechallenge with Ag-pulsed, professional APC. These data suggest that anergy and apoptosis can be separated as consequences of partial T cell signaling.

Dissociation of T cell anergy from apoptosis by blockade of Fas/Apo-1 (CD95) signaling

Induction of anergy and deletion due to apoptosis are two of the mechanisms involved in peripheral tolerance. To clarify the relationship between these two phenomena we have used an in vitro system of T cell Ag presentation. The recognition of Ag displayed by MHC class II-expressing T cells (T-APC) induces partial signals in Ag-specific T cell clones. This leads to a blunted intracellular calcium flux, and the T cells become unable to proliferate in response to further challenge with professional APC. These T cells are unable to produce IL-2, but retain the ability to release IL-4. In the present study, we report that for some T cell clones, the predominant outcome of Ag recognition on T cells is cell death. For susceptible T cell clones, the number of cells that die is proportional to the peptide concentration. This cell death resulted from Fas/Apo-1 (CD95)/Fas-ligand interactions between the T cells, in that Fas ligand expression was detected following overnight culture of T cells with T-APC and neutralizing anti-CD95 Ab protected from death. Most notably, following anti-CD95-mediated protection from apoptosis, the rescued T cells remained unable to respond to rechallenge with Ag-pulsed, professional APC. These data suggest that anergy and apoptosis can be separated as consequences of partial T cell signaling.

Gene activating and proapoptotic potential are independent properties of different CD4 epitopes

CD4 engagement triggers an early signaling cascade which initiates late events such as transcription factor activation. The outcome of CD4 engagement is T-cell commitment to alternative, dramatically different fates, such as activation and apoptosis. We have tested a panel of anti-CD4 mAbs specific for different CD4 epitopes, as well as HIV-1 gp120, for the capacity to activate crucial early events such as enhancement of p56(lck) kinase activity and Shc phosphorylation. The same CD4 epitopes were characterized for their capacity both to deliver a gene activating signal and to program T-cells to activation dependent death. No correlation could be found between capacity of specific CD4 epitopes to deliver a gene activating signal and capacity to prime T-cells to apoptosis, suggesting that gene activating and proapoptotic potential are independent functions of CD4 epitopes. Furthermore, while triggering of the calcium pathway appears critical in NF-AT activation, optimal p56(lck) activation and Shc phosphorylation might be required for initiation of the apoptotic pathway.

Cutting edge: CD4+ T cells kill CD8+ T cells via Fas/Fas ligand-mediated apoptosis

To explore the possibility that CD4+ T cells, described to mediate the elimination of themselves or B lymphocytes, could also mediate the elimination of CD8+ T cells, we analyzed apoptotic phenomena in cocultures of CD4+ and CD8+ autologous T cell lines. The data show that CD8+ T cells were lysed by activated CD4+ helper T cells by a Fas/FasL-mediated mechanism. CD4+ T cells were not lysed by activated CD8+ T cells, although Fas and FasL were equally expressed and anti-Fas Abs induced apoptosis in both CD4+ and CD8+ T cell populations. The results allowed us to speculate that CD4+ T cells not only help CD8+ T lymphocytes to mature into effector killer cells and to sustain this function but can also limit their growth.

Cutting edge: CD4+ T cells kill CD8+ T cells via Fas/Fas ligand-mediated apoptosis

To explore the possibility that CD4+ T cells, described to mediate the elimination of themselves or B lymphocytes, could also mediate the elimination of CD8+ T cells, we analyzed apoptotic phenomena in cocultures of CD4+ and CD8+ autologous T cell lines. The data show that CD8+ T cells were lysed by activated CD4+ helper T cells by a Fas/FasL-mediated mechanism. CD4+ T cells were not lysed by activated CD8+ T cells, although Fas and FasL were equally expressed and anti-Fas Abs induced apoptosis in both CD4+ and CD8+ T cell populations. The results allowed us to speculate that CD4+ T cells not only help CD8+ T lymphocytes to mature into effector killer cells and to sustain this function but can also limit their growth.

Influence of thermal and dietary stress on immune response of rabbits

With the aim of analyzing the effects of prolonged thermal stress or food intake reduction on lymphoid cell proliferation and antibody synthesis, New Zealand White (NZW) male rabbits, both immunized and nonimmunized with Mycobacterium tuberculosis, were kept in individual cages for 24 d at controlled climatic conditions. Both immunized and nonimmunized rabbits were divided into two experimental groups and one control group. The thermal-stressed rabbits (TS) were exposed to a room temperature of 33.5 degrees C, and dietary restricted rabbits (DR) were pair-fed on the basis of the average feed intake of the TS groups and maintained at a room temperature of 18.0 degrees C. The control group (Ctr) was maintained at 18.0 degrees C and was given ad libitum access to feed. All rabbits were maintained at relative humidity 62 +/- 5%. Sera and peripheral blood mononuclear cells (PBMC) were isolated from blood samples collected on d 0, 6, 12, and 24. Sera were used for determining total proteins and immunoglobulins (Ig) specific or not to mycobacterial antigens. Antibodies to heat-shock protein (HSP) were also determined. The PBMC were used to measure cell proliferation and in vitro Ig synthesis. Both experiments in vivo and in vitro suggest that thermal stress and dietary restriction severely affect the immune cell functions. In fact, both stress treatments decreased the capacity of PBMC to proliferate and inhibited the differentiation of B lymphocytes in antibody-secreting cells. However, a recovery of immune cell functions was only observed in vivo after 12 d of treatment, suggesting that other defensive mechanisms may come into play in vivo. Sera collected from both TS and DR rabbits after 24 d presented antibodies to HSP70, suggesting that the analysis of anti-HSP antibodies could represent a useful indicator to reveal the effects of different stress effectors regardless of the nature of the stress.

Differential effect of alpha-tocopherol and ascorbate on oxidative injury induced in immune cells by thermal stress

As immune cells are often subjected to hyperthermia that can easily occur either after intense and/or prolonged exercise or during defense against pathogens, in this paper we analysed whether superoxide anion production occurred in lymphocytes exposed to high temperature and, consequently, if antioxidants could exert any protective function. The results demonstrated that an increase of superoxide anion was induced in rabbit lymphocytes exposed to 42 degrees C for 1h, although cell viability was no affected. However, suppression of either Pokeweed mitogen (PWM)-driven cell proliferation, or immunoglobulin production or IL-2 synthesis was observed. To evaluate the capacity of antioxidants to restore the immune suppressed responses, two vitamins, alpha-tocopherol and ascorbic acid, were added to PWM-stimulated cultures following heat treatment. The data demonstrated that alpha-tocopherol was able to totally abrogate the inhibitory effects mediated by thermal stress, while ascorbic acid did not give any protective results.

Natural killer clones recognize specific soluble HLA class I molecules

Enhancement of major histocompatibility complex (MHC) class I expression leads to protection from natural killer (NK) cell recognition in several systems. MHC class I gene products are released from the cell surface and can be found in sera as soluble forms. To investigate the possible immunoregulatory role of soluble HLA (sHLA) in NK cell-target recognition, several sHLA antigens were studied for their ability to induce NK cell cytotoxicity modulation. NK cell-target recognition was inhibited by the addition of sHLA during the cytotoxicity assay. Our results indicate that sHLA molecules can down-regulate NK killing at the effector level. Moreover, different NK clones are able to specifically recognize different sHLA antigens. Kp43 molecules seem to be involved in the NK recognition of sHLA-B7.

Ligation of either CD2 or CD28 rescues CD4+ T cells from HIV-gp120-induced apoptosis

Temporal or quantitative imbalance in signals delivered to T cells via T cell antigen receptor (TCR), the CD4 co-receptor, and accessory molecules can lead to anergy, apoptosis, or both. This has been observed following ligation of CD4 by HIV gp120 prior to TCR occupancy. The ability of molecules such as CD2 and CD28, interacting with their ligands LFA-3 and B7, to provide signals that protect T cells from the induction of anergy, has been reported. Here, we demonstrate that ligation of CD2 and CD28 in conjunction with TCR occupancy rescue T cells that have been programmed for apoptotic death by prior CD4 ligation to gp120. This appears to be the result of augmented interleukin-2 and interleukin-4 release by the T cells following these molecular interactions. In conclusion, our results suggest that an impairment of antigen-presenting accessory cell functions could favor gp120-mediated apoptosis in HIV-uninfected cells.

Differential susceptibility to monomeric HIV gp120-mediated apoptosis in antigen-activated CD4+ T cell populations

To support the hypothesis that indirect mechanisms mediated by viral products like the HIV envelope glycoprotein gp120 could be responsible for T lymphocyte depletion in HIV infection, we developed a system in which the impairment of T cell functions could be investigated in vitro. In particular, we characterized the conditions that allow T lymphocytes repeatedly stimulated with an antigen to be sensitive or resistant to gp120-mediated apoptotic signals. To achieve this goal, a panel of antigen-specific CD4+ T cell clones and primary CD4+ T lymphocytes were treated for 2 and 18 h with saturating amounts of monomeric gp120 (without cross-linking with specific antibodies) and antigen-driven T cell proliferation and apoptosis were analyzed. We show that monomeric gp120 induces apoptosis only in T lymphocytes repeatedly stimulated with the antigen, that primary T lymphocytes are resistant to programmed cell death mediated by monomeric gp120, but are sensitive to anti-CD4 antibodies, and that gp120-mediated apoptosis is dependent on the period of time between the binding of gp120 to CD4 and the encounter with antigen. To investigate the different susceptibility to gp120 induced apoptosis of primary CD4+ and T cell clones further, the number of membrane CD4 molecules and their affinity for gp120, together with Bcl-2 and Fas expression, were studied. Our data suggest that a down-modulation of membrane CD4 together with high expression of the Bcl-2 gene and protein characterizes the susceptibility to apoptosis of gp120-treated cells. In conclusion, our results define the phenotypic features of T cells susceptible to HIV gp120-induced apoptosis and demonstrate that the same clonotype, depending on the activation state, may present a differential sensitivity to apoptosis induction.

Identification of a novel HLA-B27 subtype by restriction analysis of a cytotoxic gamma delta T cell clone

Seven HLA-B27 alleles are known, which share the same allospecificity, but differ by one to six amino acid substitutions. Herein, we describe a novel HLA-B27 allele, provisionally named B27-ci, which is expressed by an individual from whom a B27-restricted gamma delta T cell clone has been derived. This clone recognizes B cell lines from the proband and all of the other B27-positive members of the family, but does not lyse B cell lines that express other HLA-B27 alleles. The amino acid sequence deduced from three B27-ci cDNA clones was found to differ from the B*2705 sequence by one amino acid substitution (Asp to His) in position 116 of the alpha 2 domain. This position has been shown to lie in the floor of the F pocket, where it plays a key role in determining the nature of the amino acid side chain that will fit into this pocket. Moreover, the fact that the clone described here possesses a TCR-gamma delta indicates that this subset of cells not only can be HLA-restricted, but also can finely discriminate among classical class I molecules.

Identification of a novel HLA-B27 subtype by restriction analysis of a cytotoxic gamma delta T cell clone

Seven HLA-B27 alleles are known, which share the same allospecificity, but differ by one to six amino acid substitutions. Herein, we describe a novel HLA-B27 allele, provisionally named B27-ci, which is expressed by an individual from whom a B27-restricted gamma delta T cell clone has been derived. This clone recognizes B cell lines from the proband and all of the other B27-positive members of the family, but does not lyse B cell lines that express other HLA-B27 alleles. The amino acid sequence deduced from three B27-ci cDNA clones was found to differ from the B*2705 sequence by one amino acid substitution (Asp to His) in position 116 of the alpha 2 domain. This position has been shown to lie in the floor of the F pocket, where it plays a key role in determining the nature of the amino acid side chain that will fit into this pocket. Moreover, the fact that the clone described here possesses a TCR-gamma delta indicates that this subset of cells not only can be HLA-restricted, but also can finely discriminate among classical class I molecules.

Different regions of the N-terminal domains of HLA-DR1 influence recognition of individual peptide-DR1 complexes

The contributions of individual amino acids in the polymorphic beta chain and the conserved alpha chain of HLA-DR1 to influenza HA-specific DR1-restricted and anti-DR1 allospecific T-cell recognition were analyzed. The genes encoding HLA-DR1 were subjected to site-directed mutagenesis in order to introduce single amino acid substitutions at 12 positions in the beta 1 domain and 11 positions in the alpha 1 domain. The beta 1-domain substitutions were all at polymorphic positions and introduced residues that are found in DR4 alleles. The amino acids introduced into the DR alpha 1 domain were based on the sequences of other human and mouse class II alpha chains. The responses of 12 DR1-restricted T-cell clones specific for two peptides of HA and seven anti-DR1 allospecific clones were studied. Substitutions at positions that point up from and into the peptide-binding site in the third variable region of the beta 1-domain alpha-helix caused substantial reduction in the responses of all of the clones. Substitutions at multiple positions in the beta 1-domain floor and in the alpha 1 domain influenced the anti-DR1 responses of the alloreactive and of the HA100-115-specific T-cell clones. In contrast, very few changes outside of the beta 1 domain third variable region affected the responses of the HA306-324-specific DR1-restricted T-cell clones. These results suggest that a surprisingly limited region of the HLA-DR1 molecule is critically involved in T-cell recognition of HA306-324 by DR1-restricted T cells. However, the susceptibility of the HA100-115-specific and the anti-DR1 allospecific T-cell clones to substitutions at multiple positions in both N-terminal domains shows that the response to DR1-HA306-324 is unusual and may reflect the promiscuity with which this peptide binds to HLA-DR molecules.

Analysis of susceptibility of mature human T lymphocytes to dexamethasone-induced apoptosis

We present evidence that dexamethasone (Dex), a synthetic glucocorticosteroid, causes apoptosis in mature human T cells, similarly to what has been reported for murine T lymphocytes. Human T cell clones and short-term activated T lymphocytes treated with Dex show the characteristic pattern of apoptotic cells, such as hypodiploid nuclei, chromatin condensation and DNA fragmentation into oligonucleosomal fragments. However, Dex susceptibility of T cells to apoptosis is cell cycle-dependent. The progression in the proliferative cell cycle (G1 versus S) rescues Dex-treated T cells from apoptosis. Moreover, occupancy of the T cell receptor reverses Dex-induced apoptotic phenomena. These observations suggest that glucocorticoids contribute to the regulation of the proliferative or the suicidal response of antigen-activated human T cells.

Dexamethasone-dependent modulation of human lymphoblastoid B cell line through sphingosine production

The relationship between dexamethasone-dependent changes in intracellular sphingosine levels, energy and phospholipid metabolism have been investigated by 31P-NMR spectroscopy and high-performance liquid chromatography. The cellular functions have been evaluated by cellular growth and immunoglobulin M secretion (IgM). Significant increases in intracellular phosphorylcholine (PCho), extracellular choline (Cho), and endogenous sphingosine levels were observed only at 30 min incubation with dexamethasone. These results confirmed a sphingosine-dependent hydrolysis of choline-linked phospholipids (Miccheli, A., Ricciolini, R., Piccolella, E., Delfini, M. and Conti, F. (1991) Biochim. Biophys. Acta 1093, 29-35). Furthermore, no significant variations were evidenced at hours 1, 2, 6 and 18 of incubation. Dexamethasone causes an inhibition of cellular growth and IgM secretion as well as the sphingosine treatment. The results suggest that the effect of dexamethasone may be mediated by endogenous sphingosine production in Epstein-Barr virus transformed B lymphocytes.

Dexamethasone-dependent modulation of cholesterol levels in human lymphoblastoid B cell line through sphingosine production

The effect of dexamethasone on lipid composition of Epstein-Barr virus transformed human B lymphocytes have been investigated by 31P- and 1H-NMR spectroscopy and compared to the effects due to exogenous sphingosine treatment. Furthermore, the effects of dexamethasone and sphingosine on membrane structure was evaluated by fluorimetry. No significant changes were evidenced in phospholipid composition and in the ratio of unsaturated to total fatty-acid chains. A significant increase in total cholesterol levels was evident at 30 min incubation with dexamethasone or sphingosine; a parallel increase in DPH polarization at 30 min was also demonstrated. TMA-DPH intensity measurements suggest a slowing of vesicular intracellular traffic due to the treatment. The results suggest a dexamethasone- and sphingosine-dependent inhibition of intracellular cholesterol transport.

Characterization of an IS-like element from Mycobacterium tuberculosis

A DNA sequence, present in members of the Mycobacterium tuberculosis complex, has been identified and characterized. The distribution of this DNA sequence among mycobacterial species was analysed by DNA hybridization and PCR experiments. As the sequence was detected only in bacteria belonging to the M. tuberculosis complex, it may be useful for the rapid discrimination of mycobacteria. Interestingly, the sequence has some characteristics of an insertion element (IS) and codes for a hypothetical protein with significant homologies to proteins encoded by several IS elements of other organisms, namely IS427 and IS869 from Agrobacterium tumefaciens, IS402 from Pseudomonas cepacia, Tn4811 from Streptomyces lividans and ISRm4 from Rhizobium meliloti. Together, these elements form a previously unrecognized family of transposable elements. This finding suggests the possibility of horizontal gene transfer between pathogenic mycobacteria and other organisms including Gram-negative plant-pathogenic bacteria.

Antibody repertoire against the A60 antigen complex during the course of pulmonary tuberculosis

The A60 antigen complex is a Mycobacterium bovis (BCG) highly immunodominant antigen containing both B and T-cell epitopes. Clinical-serological studies show that elevated anti-A60 titres are present during tuberculosis. We wished to analyze in detail antibody responses against A60 components during the course of tuberculosis. A mixed longitudinal study was designed including individuals at the onset of tuberculosis, during treatment and after resolution of the disease. The anti-A60 repertoire was analyzed using a western blot assay with A60 as the antigen. While PPD- normals recognized only the 65 kDa heat shock protein (HSP), PPD+ normal individuals displayed low levels of anti-A60 antibodies against dominant antigens. There were immunoglobulin M (IgM) and immunoglobulin G (IgG) consistent with response to a latent infection. Onset tuberculosis was characterized by IgM and IgG antibodies against 52 to 28 kDa antigens; IgM response being limited to earlier phases of the disease. In contrast, IgM antibodies against 25 to 14 kDa antigens appeared only 2-6 months after disease onset. The antibody repertoire of chemotherapy-treated, resolved tuberculosis was exclusively IgG in isotype, as for a memory-type response. Thus, western blot analysis with A60 identifies typical antibody patterns associated with different clinical phases of tuberculosis infection. Such approach may help in identifying new single antigens for serologic diagnosis of active tuberculosis.

Antibody repertoire against the A60 antigen complex during the course of pulmonary tuberculosis

The A60 antigen complex is a Mycobacterium bovis (BCG) highly immunodominant antigen containing both B and T-cell epitopes. Clinical-serological studies show that elevated anti-A60 titres are present during tuberculosis. We wished to analyze in detail antibody responses against A60 components during the course of tuberculosis. A mixed longitudinal study was designed including individuals at the onset of tuberculosis, during treatment and after resolution of the disease. The anti-A60 repertoire was analyzed using a western blot assay with A60 as the antigen. While PPD- normals recognized only the 65 kDa heat shock protein (HSP), PPD+ normal individuals displayed low levels of anti-A60 antibodies against dominant antigens. There were immunoglobulin M (IgM) and immunoglobulin G (IgG) consistent with response to a latent infection. Onset tuberculosis was characterized by IgM and IgG antibodies against 52 to 28 kDa antigens; IgM response being limited to earlier phases of the disease. In contrast, IgM antibodies against 25 to 14 kDa antigens appeared only 2-6 months after disease onset. The antibody repertoire of chemotherapy-treated, resolved tuberculosis was exclusively IgG in isotype, as for a memory-type response. Thus, western blot analysis with A60 identifies typical antibody patterns associated with different clinical phases of tuberculosis infection. Such approach may help in identifying new single antigens for serologic diagnosis of active tuberculosis.

Subtelomeric as well as telomeric sequences are lost from chromosomes in proliferating B lymphocytes

B lymphocytes purified from peripheral blood can be normally cultured in vitro for only one doubling. They can undergo an unlimited number of cell divisions after transformation with a DNA tumor virus such as the Epstein-Barr virus. We have shown that the terminal restriction fragments of virus transformed B lymphocytes are shortened in the course of proliferation and that this process is accompanied by structural modifications. We have identified the sequences that are lost during the shortening process by hybridization to the canonical human telomeric simple repeat TTAGGG, to other simple sequences that are found at the ends of human chromosomes, and to a human subtelomeric sequence. We have observed that by 20 doublings over half the TTAGGG sequences, but few or no TGAGGG sequences, are lost from the TRFs. The subtelomeric sequence was removed from most of the TRFs on which it was present. The implications that these observations have on the problems of cell senescence and oncology are discussed.

Mycobacterial antigen complex A60-specific T-cell repertoire during the course of pulmonary tuberculosis

The Mycobacterium bovis antigen complex A60 is known to be immunodominant in tuberculosis and to have a protective effect against experimental infection in vitro and in vivo. To identify immunodominant and possibly protective antigens in pulmonary tuberculosis, the T-cell repertoire directed to nitrocellulose-bound fractions of A60 antigen was analyzed in active tuberculosis patients during the course of the infection and after recovery. The results show that patients infected with Mycobacterium tuberculosis acquired complete A60-T-cell reactivity only in the late phases of infection. At disease onset, patients with active tuberculosis were characterized by (i) T-cell unresponsiveness to most A60 fractions, (ii) high tumor necrosis factor alpha production, and (iii) low gamma interferon (IFN-gamma) release. Several weeks after chemotherapy, the unresponsive state disappeared and the following reverse situation was observed: (i) high blastogenic response to almost all A60 fractions, (ii) low tumor necrosis factor alpha release, and (iii) high IFN-gamma production. In addition, 60% of these patients significantly responded against seven A60 fractions (61 to 58, 56 to 53, 49 to 46, 46 to 44, 35 to 33, 33 to 30, and 30 to 28 kDa), indicating that they included immunodominant antigens. Furthermore, only the fractions within the molecular mass ranges of 56 to 44 and 35 to 28 kDa induced IFN-gamma synthesis. One year after complete recovery from infection, more than 60% of past-active tuberculosis subjects had memory T cells specific for the immunodominant fractions of 61 to 58, 56 to 53, 49 to 46, and 33 to 30 kDa. Since the same fractions induced the strongest IFN-gamma production, known to exhibit antimycobacterial effects, it is suggested that these may represent the inducers of a protective immune response.

A new tripeptide, Pol 509, influences biochemical events associated with antigen presentation efficiency of PPD-specific EBV-B cells

A synthetic tripeptide (pGLU-LEU-TRP-OCH3) Pol 509, derived from snake venom, was studied directly by analyzing the interactions with synthetic lipid bilayers using NMR spectroscopy. Functional studies were also performed by measuring the effects: i), on early biochemical events (adenyl cyclase and phospholipase C activation products), intermediate (surface Ag expression) and late (DNA synthesis) parameters following B-cell activation elicited by PPD-linkage to specific membrane Ig; and ii), on the presentation of PPD to Ag-specific T-cell lines. Comparative experiments using PMA and IFN-gamma were also performed. We found that all parameters studied were affected by Pol 509 treatment. In fact, while PPD linkage to mlg reversed the balance between cAMP and IP3 existing in unstimulated EBV-B cells, Pol 509 reduced the PPD-induced accumulation of cAMP to control values and induced a further decrease of IP3 level. Pol 509-mediated decrease of these second messenger levels was accompanied by a slight increase of HLA-DR molecule expression and DNA synthesis inhibition. Furthermore, Pol 509 enhanced the efficiency of PPD presentation to T-cell lines. Taken together, these observations suggest that Pol 509, which enhances Ag presentation by modifying second messenger levels, may be considered as a new immunomodulatory drug with immunopotentiating activity.

Modulation of the free sphingosine levels in Epstein Barr virus transformed human B lymphocytes by phorbol dibutyrate

The amounts of free sphingosine in Epstein Barr virus transformed B lymphocytes (EBV-B) treated with sphingosine and phorbol-12,13-dibutyrate (PD) has been quantified by high performance liquid chromatography (HPLC). PD treatment did not affect intracellular sphingosine level, while it seems to lessen the removal of this long chain base in sphingosine-treated EBV-B cells. The previous results relative to sphingosine-dependent changes in choline-metabolite levels have to be interpreted on the basis of these results.

Modulation of human lymphoblastoid B cell line by phorbol ester and sphingosine. A 31P-NMR study

Changes in phospholipid and energy metabolism in Epstein-Barr Virus transformed B lymphocytes (EBV-B), induced by phorbol 12,13-dibutyrate (PD) and sphingosine (an inhibitor of protein kinase C), have been evaluated by 31P-NMR spectroscopy. The effects of PD and sphingosine on [3H]thymidine incorporation have also been studied. An increase in phosphorylcholine (PCho) levels has been observed in sphingosine and sphingosine + PD treated cells after 30 min of incubation, whereas no change was observed in lymphocytes incubated with PD during the same period. Extracellular choline levels increased in sphingosine treated cells but decreased in PD treated cells. Hence, a sphingosine-dependent hydrolysis of choline-linked phospholipids is suggested. A time-dependent reduction of PCho observed after 120 min PD incubation is consistent with an increase of the synthesis of choline-linked phospholipids.

Regulation of self-major histocompatibility complex reactive human T-cell clones

The proliferative response of human T-lymphocyte clones, (TLC) specific for self-major histocompatibility complex (MHC) products either alone or associated with PPD epitopes are inhibited in vitro by dexamethasone (DEX) and by a non-specific inhibitory factor(s) (nsINH) produced by PPD-activated T-cells. The inhibiting effect has been investigated by preincubating autoreactive and PPD-specific TLC with nsINH or DEX. Results obtained indicate that T-lymphocytes are the target of these two immunoregulatory molecules. Moreover, the addition of exogenous recombinant interleukin 2 (rIL-2) substantially reverses the inhibition observed in both nsINH- or DEX-treated cultures.

Identification of a 35-kilodalton Mycobacterium tuberculosis protein containing B- and T-cell epitopes

Screening of a Mycobacterium tuberculosis genomic DNA library in the lambda gt11 expression vector was carried out by using, as probes, sera from tuberculous patients and murine monoclonal antibody H61.3 recognizing a mycobacterial 35-kilodalton protein present only on the M. tuberculosis complex. The recombinant beta-galactosidase-fused protein present in the crude lysate induced the proliferation of T lymphocytes from patients with tuberculous pleuritis. As the recombinant insert contains an internal EcoRI restriction site, it was possible to identify two fragments, one proximal to the lacZ gene and 1.7 kilobases (kb) in size and the other distal to the lacZ gene and 2.2 kb in size. Southern blot analysis showed that both of them hybridized with the genomic DNA from M. tuberculosis and M. bovis but not with the DNA from other mycobacterial species. To perform extensive immunological studies, the amount of beta-galactosidase-fused protein being very low, we fused the 1.7-kb fragment to the N-terminal part of the gene coding for the DNA polymerase of bacteriophage MS2 in the expression vector pEx34. The fusion protein was partially purified, and subsequent Western blotting (immunoblotting) and T-cell proliferation experiments confirmed the presence of B- and T-cell mycobacterial epitopes. Furthermore, to isolate the chromosomal region containing the 35-kilodalton gene, we constructed another mycobacterial genomic library in the lambda 2001 vector by cloning 15 to 20 kb of foreign DNA. Screening of this library was carried out by using 1.7- and 2.2-kb recombinant fragments as probes. Restriction maps of some clones isolated were determined.

Increased autoreactive T cell frequency in tuberculous patients

The development of putative self-MHC-reactive T cells and their precursor frequency was estimated in peripheral blood lymphocyte cultures stimulated in vitro with PPD. The role of foreign antigen in the generation of self-MHC-reactive T cells in vivo was analyzed by comparing the frequency of autoreactive T cells in the peripheral blood of tuberculous patients with that observed in healthy individuals. It was found that PPD in vitro and Mycobacterium tuberculosis infection in vivo increased substantially the generation of autoreactive T cells. Autoreactive T cell clones were shown (1) to recognize self MHC class II products; (2) to release gamma interferon in the absence of exogenous antigen, and (3) to express autocytotoxic activity. All these findings suggest that self-MHC-reactive T cells may be involved in the inflammatory response to M. tuberculosis.

Limiting dilution analysis of T cell unresponsiveness to mycobacteria in advanced disseminated tuberculosis

Peripheral blood mononuclear cells from patients with advanced disseminated tuberculosis (Dis-TB) do not respond to purified protein derivative (PPD) measured as cell proliferation, lymphokine production and interleukin (IL)-2 receptor (Tac antigen) expression. Limiting dilution analysis revealed "multi-hit" curves and low frequencies of PPD-reactive T cells in cultures of Dis-TB, and "single-hit" curves and high frequencies of PPD-reactive T cells in cultures of patients with localized form of pulmonary tuberculosis. Moreover, a strict relationship between Tac antigen expression and ability of exogenous IL-2 to enhance bulk culture cell proliferation was observed in Dis-TB patients.

Epstein-Barr virus-transformed B cells process and present Mycobacterium tuberculosis particulate antigens to T-cell clones

We have analyzed the presentation of mycobacterial antigens by Epstein-Barr virus-transformed human B (EBV-B) cells to mycobacteria-specific T-cell clones and lines, and to purified resting T cells. EBV-B cells were able to process and present not only soluble forms of antigen, such as PPD and the expressate preparation of M. tuberculosis strain H37Rv, but also particulate forms of antigen, such as whole mycobacterial H37Rv or M. bovis organisms. Electron microscopy studies demonstrated the capacity of EBV-B cells to phagocytose mycobacterial cells in 18 hr and pulsing experiments confirmed that an 18-hr of incubation is required for an efficient processing and presentation of mycobacterial determinants to T cells. The processing of whole-H37Rv particulate antigen by EBV-B cells was inhibited by the lysosomotrophic compound chloroquine and by high doses of irradiation. Finally, the analysis of the presentation of soluble and particulate mycobacterial antigens by PPD-positive and PPD-negative EBV-B cell clones has shown a preferential presentation of both forms of antigen by PPD-positive EBV-B clones.

Immunology of tuberculosis: new directions in research

Tuberculosis is still one of the major health problems in almost all over the world. Thus, new directions in basic and applied research on tuberculosis are under investigation. In this review we have provided recent data obtained in our laboratories on three main aspects of the immunology of tuberculosis, namely: i. the role of B lymphocytes in the processing and presentation of Mycobacterium tuberculosis antigens to T cells; ii. the activation and characterization of mycobacterial-specific T cell clones; iii. the T cell regulation of the immune response to M. tuberculosis. The analysis of the antigenic determinants of M. tuberculosis relevant in the antimycobacterial immunity is the major goal of the WHO programme on the immunology of tuberculosis. In fact, the attempt to develop a second generation vaccine against this microorganism is now possible by analyzing recombinant genomic DNA libraries of M. tuberculosis with monoclonal antibodies and T cell clones. In the near future, the identification of epitopes recognized by mycobacterial-specific T cells with helper, cytotoxic and suppressor functions will allow the preparation of recombinant and synthetic vaccines effective in the control of this disease.

Regulation of immune interferon production during the response to soluble microbial antigens

Human peripheral blood mononuclear cells (PBMC) stimulated in vitro with purified protein derivative (PPD) or with a Candida albicans polysaccharide extract (MPPS) released immune interferon (IFN) and interleukin 2 (IL-2). Kinetic studies showed a biphasic production of IFN with maximum levels at days 3-4 and days 5-6 of culture. In contrast, the IL-2 production is only observed at days 2-3 of culture. The relationship between IFN and IL-2, analysed both in responder and nonresponder PBMC cultures, showed that the early peak of IFN production appears to be IL-2 independent whereas the second peak seems strictly related to the presence of IL-2 culture. Furthermore, monoclonal antibodies against class I and class II products of the major histocompatibility complex (MHC) inhibited IFN production when added at the beginning of culture, whereas only anti-class I antibodies interfered with the release of IFN when added to antigen-primed lymphocytes.