- Email: [email protected]
Host pathogen interactions
IMW!JNE
SYSTEM PHYSIOLOGY
(APC : only dendritic cells and macrophages) or into C5+ nude recipients (APC: B cells in addition to dendritic cells and macrophages). In the first case, a massive expansion of the injected T cells was observed, whereas in the second case, only a few of the injected T cells could be recovered. Apparently, B cells are unable to trigger responses in naive T cells and may even induce anergy in this system. Finally, systems using immunoglobulins as autoantigens were discussed. In anti-IgG2ab TCR transgenic mice, transgenic T cells were not deleted in the thymus (Francesca Granucci, Milano). In the periphery, they cause the disappearance of IgG2a b-secreting cells. After immunization with S. typhimurium, the IgG2ab serum levels remained much lower in trans-
AND PATHOLOGY
411
genie mice than in controls and a 5-fold increase in IgEb was observed. It was proposed that anti-IgG2ab T cells may exist in normal individuals and play a regulatory role in the control of the IgG2a to IgE class switch. The influence of the heavy chain class on the immunogenicity of an idiotype was demonstrated by Kristian Hannestad (Tromso). Whereas immunization with an IgM monoclonal antibody elicited a vigorous humoral response against the idiotype, multiple challenges with the IgG switch variant failed to induce such a response. It was therefore suggested that dominant B-cell clones can pre-empt antiidiotypic antibody responses against themselves by early switching from IgM to IgG secretion, before immunogenic IgM antibodies have had time to activate antiidiotypic B cells.
Host pathogen interactions Chairperson
: A. McMichael
The main focus of this session was evasion of the immune response by viruses. It is becoming increasingly clear that DNA viruses have evolved elegant strategies to evade the immune response, and in particular, interfere with antigen processing or presentation by class I MHC molecules. Herpes viruses seem to specialize in interfering with antigen processing, whereas other DNA viruses such as the pox viruses have evolved alternative strategies. The dsDNA genome of vaccinia virus is large and encodes many enzymes and virulence factors. The virus has a number of mechanisms which interfere with Immune responses (G. Smith, Oxford). These include proteins that affect steroid hormone synthesis or block interferon, cytokines or complement. The anticytokine
(Oxford)
factors include soluble receptors for interferon-y, interferon alp, ILlp and TNF. Deletion of these genes does not always attenuate the virus. For instance, deletion of the ILlPR from vaccinia strain WR caused a greater weight loss and a febrile response in infected mice, while wild-type WR and other strains which express ILlPR failed to induce fever. On the other hand, deletion of the WR IFN-odPR attenuated the virus. Notably this receptor is expressed by all other orthopox viruses tested except vaccinia strain Lister, which was one of the safest smallpox vaccine strains. There are three main families of herpes viruses, alpha, beta and gamma. These viruses persist despite a vigorous immune response, and some are neurotropic. They have devised a number of evasion strategies to avoid immune
412
SUMMARY
responses, in particular cytotoxic T-lymphocyte responses. U. Koszinowski (Heidelberg) showed that murine cytomegalovirus utilises a mechanism where an early gene product arrests the maturation of class I molecules in the ER, as judged by sensitivity to endoglycosidase-H. The responsible gene can be deleted from the virus, restoring normal class I molecule maturation. D. Johnson (Toronto) described the way herpes simplex virus evades T-cell responses. The immediate early gene ICP47 interferes with TAP 1,2 peptide transporters. This results in a low level of MHC class I expression and failure to present antigen to CTL. Johnson and colleagues s.howed that ICP47, a small cytosolic protein binds to TAP 1 and 2 complexes and interferes with their ability to transport peptides into the endoplasmic reticulum. The effect is much greater in human cells compared to murine cells. Expression of ICP47 in an adenovirus vector results in poor presentation of virus antigens to CTL. Epstein Barr virus uses yet another strategy. Burkitt lymphoma lines and some chronically infected B cells in humans express only the EBNA 1 antigen. Despite intensive investigation, no cytotoxic T-lymphocyte response has been described against EBNAl. M. Masucci (Stockholm) investigated the gly-ala repeat sequence in EBNA 1. When an EBNA 4 epitope, presented by HLA-All, was inserted into the EBNA 1 sequence, vaccinia expressing cells failed to present the peptide. However, when the gly-ala repeat sequence was deleted from such constructs, presentation proceeded normally. When the gly-ala sequence was inserted into EBNA 4, the EBNA 4 epitope was not presented. She argued therefore that the gly-ala repeat interferes with antigen processing. This effect only acts as cis and is therefore likely to be interfering with the processing of the EBNA 1 protein itself, in contrast to the broadly non-specific effect of the ICP47 interference with processing. She also described another strategy of epitope mutation where some strains of EBV in areas where HLAAll is highly prevalent have mutated the dominant peptide epitope sequence so that it no longer binds to HLA-A 11.
OF SESSIONS
Another form of virus avoidance of control by CTL is demonstrated by acute infection with high doses of the fast-replicating strains of LCMVDOC, which activate and exhaust the whole CTL response. Viral antigen persists in the thymus and mice are subsequently tolerized. M. van den Broek (Zurich) showed that administration of IL12 made mice less susceptible to high dose exhaustion. Mice, such as the BALB/c strain, that tend towards Th2 responses are more likely to show this effect, but it can be prevented by coadministration of anti-IL4 antibody. Mouse mammary tumour virus uses the immune response to infect. S. Luther (Lausanne) described how a superantigen activates Th, according to strain-specific TCR VP specificities. These T cells activate B cells to divide and differentiate, enabling the virus to survive within the B-cell compartment. In addition, the antibody response prevents superinfection. Measles virus uses CD46 P. Rivailler (Lyon) described this molecule. This gave an investigation of virus capture tation. An in vivo model may
to infect cells and mice transgenic for in vitro model for and antigen presennow be close.
The cytotoxic T-lymphocyte response to HIV1 shows a number of unusual features. One is the very high level of CTL activity present in peripheral blood lymphocytes. P. Moss (Oxford) showed that in some patients, individual CTL clones could be present at levels of l-2% of CD8 T cells. This was shown by using anchor PCR to amplify T-cell receptor cDNA and then to probe specifically for the unique VP CDR3 region of peptide specific clones. These high frequencies were found in 3 different patients, two HLA-B27 gag p24-specific clones and one HIV pol HLA2specific clone. He suggested that these very high frequencies of HIV-specific CTL may result from overstimulation by high viral antigen loads and that they may result in exhaustion of particular immune responses in due course. Variability in HIV poses problems for the CTL control of this virus infection. A. McMichael (Oxford) described the considerable variation in epitopes recognized by HIV-specific CTL presented by HLA molecules such as HLA-B8. He argued that a consequence of this variation is a
IMMUNE
SYSTEM
PHYSIOLOGY
broadening of the immune response from the normal immunodominant pattern, focusing on a very small number of epitopes, to broad responses to multiple epitopes. Thus, whereas influenza-specific and EBV-specific responses tend to focus on very few epitopes, sometimes with single dominant receptors, anti-HIV responses are usually broad. However, in a few patients, CTL responses may focus on highly conserved sequences and therefore be restricted in heterogeneity. He suggested that the failure of CTL to control HIV infection may result from the continuous escape of HIV from CTL control, only partly controlled by new T-cell responses. As the infection progresses, CD4 T-cell help declines and the infected patient may have difficulty in
AND
PATHOLOGY
413
generating new CTL responses to variant virus epitopes and in maintaining old CTL responses. Overall, the session highlighted a number of mechanisms of immune escape from CTL control of virus infections. The strategies demonstrate the importance of CTL in controlling virus infections. Often, escape is associated with persistence of the virus. In many cases, the stable equilibrium is reached where the virus is largely controlled, although persisting but not causing harm (e.g. HSV, CMV, EBV). However, in some cases, such an equilibrium is not reached (HIV1 and HIV2). Understanding these virus escape strategies gives some rational basis on which to develop immunotherapies.
SYSTEM PHYSIOLOGY
(APC : only dendritic cells and macrophages) or into C5+ nude recipients (APC: B cells in addition to dendritic cells and macrophages). In the first case, a massive expansion of the injected T cells was observed, whereas in the second case, only a few of the injected T cells could be recovered. Apparently, B cells are unable to trigger responses in naive T cells and may even induce anergy in this system. Finally, systems using immunoglobulins as autoantigens were discussed. In anti-IgG2ab TCR transgenic mice, transgenic T cells were not deleted in the thymus (Francesca Granucci, Milano). In the periphery, they cause the disappearance of IgG2a b-secreting cells. After immunization with S. typhimurium, the IgG2ab serum levels remained much lower in trans-
AND PATHOLOGY
411
genie mice than in controls and a 5-fold increase in IgEb was observed. It was proposed that anti-IgG2ab T cells may exist in normal individuals and play a regulatory role in the control of the IgG2a to IgE class switch. The influence of the heavy chain class on the immunogenicity of an idiotype was demonstrated by Kristian Hannestad (Tromso). Whereas immunization with an IgM monoclonal antibody elicited a vigorous humoral response against the idiotype, multiple challenges with the IgG switch variant failed to induce such a response. It was therefore suggested that dominant B-cell clones can pre-empt antiidiotypic antibody responses against themselves by early switching from IgM to IgG secretion, before immunogenic IgM antibodies have had time to activate antiidiotypic B cells.
Host pathogen interactions Chairperson
: A. McMichael
The main focus of this session was evasion of the immune response by viruses. It is becoming increasingly clear that DNA viruses have evolved elegant strategies to evade the immune response, and in particular, interfere with antigen processing or presentation by class I MHC molecules. Herpes viruses seem to specialize in interfering with antigen processing, whereas other DNA viruses such as the pox viruses have evolved alternative strategies. The dsDNA genome of vaccinia virus is large and encodes many enzymes and virulence factors. The virus has a number of mechanisms which interfere with Immune responses (G. Smith, Oxford). These include proteins that affect steroid hormone synthesis or block interferon, cytokines or complement. The anticytokine
(Oxford)
factors include soluble receptors for interferon-y, interferon alp, ILlp and TNF. Deletion of these genes does not always attenuate the virus. For instance, deletion of the ILlPR from vaccinia strain WR caused a greater weight loss and a febrile response in infected mice, while wild-type WR and other strains which express ILlPR failed to induce fever. On the other hand, deletion of the WR IFN-odPR attenuated the virus. Notably this receptor is expressed by all other orthopox viruses tested except vaccinia strain Lister, which was one of the safest smallpox vaccine strains. There are three main families of herpes viruses, alpha, beta and gamma. These viruses persist despite a vigorous immune response, and some are neurotropic. They have devised a number of evasion strategies to avoid immune
412
SUMMARY
responses, in particular cytotoxic T-lymphocyte responses. U. Koszinowski (Heidelberg) showed that murine cytomegalovirus utilises a mechanism where an early gene product arrests the maturation of class I molecules in the ER, as judged by sensitivity to endoglycosidase-H. The responsible gene can be deleted from the virus, restoring normal class I molecule maturation. D. Johnson (Toronto) described the way herpes simplex virus evades T-cell responses. The immediate early gene ICP47 interferes with TAP 1,2 peptide transporters. This results in a low level of MHC class I expression and failure to present antigen to CTL. Johnson and colleagues s.howed that ICP47, a small cytosolic protein binds to TAP 1 and 2 complexes and interferes with their ability to transport peptides into the endoplasmic reticulum. The effect is much greater in human cells compared to murine cells. Expression of ICP47 in an adenovirus vector results in poor presentation of virus antigens to CTL. Epstein Barr virus uses yet another strategy. Burkitt lymphoma lines and some chronically infected B cells in humans express only the EBNA 1 antigen. Despite intensive investigation, no cytotoxic T-lymphocyte response has been described against EBNAl. M. Masucci (Stockholm) investigated the gly-ala repeat sequence in EBNA 1. When an EBNA 4 epitope, presented by HLA-All, was inserted into the EBNA 1 sequence, vaccinia expressing cells failed to present the peptide. However, when the gly-ala repeat sequence was deleted from such constructs, presentation proceeded normally. When the gly-ala sequence was inserted into EBNA 4, the EBNA 4 epitope was not presented. She argued therefore that the gly-ala repeat interferes with antigen processing. This effect only acts as cis and is therefore likely to be interfering with the processing of the EBNA 1 protein itself, in contrast to the broadly non-specific effect of the ICP47 interference with processing. She also described another strategy of epitope mutation where some strains of EBV in areas where HLAAll is highly prevalent have mutated the dominant peptide epitope sequence so that it no longer binds to HLA-A 11.
OF SESSIONS
Another form of virus avoidance of control by CTL is demonstrated by acute infection with high doses of the fast-replicating strains of LCMVDOC, which activate and exhaust the whole CTL response. Viral antigen persists in the thymus and mice are subsequently tolerized. M. van den Broek (Zurich) showed that administration of IL12 made mice less susceptible to high dose exhaustion. Mice, such as the BALB/c strain, that tend towards Th2 responses are more likely to show this effect, but it can be prevented by coadministration of anti-IL4 antibody. Mouse mammary tumour virus uses the immune response to infect. S. Luther (Lausanne) described how a superantigen activates Th, according to strain-specific TCR VP specificities. These T cells activate B cells to divide and differentiate, enabling the virus to survive within the B-cell compartment. In addition, the antibody response prevents superinfection. Measles virus uses CD46 P. Rivailler (Lyon) described this molecule. This gave an investigation of virus capture tation. An in vivo model may
to infect cells and mice transgenic for in vitro model for and antigen presennow be close.
The cytotoxic T-lymphocyte response to HIV1 shows a number of unusual features. One is the very high level of CTL activity present in peripheral blood lymphocytes. P. Moss (Oxford) showed that in some patients, individual CTL clones could be present at levels of l-2% of CD8 T cells. This was shown by using anchor PCR to amplify T-cell receptor cDNA and then to probe specifically for the unique VP CDR3 region of peptide specific clones. These high frequencies were found in 3 different patients, two HLA-B27 gag p24-specific clones and one HIV pol HLA2specific clone. He suggested that these very high frequencies of HIV-specific CTL may result from overstimulation by high viral antigen loads and that they may result in exhaustion of particular immune responses in due course. Variability in HIV poses problems for the CTL control of this virus infection. A. McMichael (Oxford) described the considerable variation in epitopes recognized by HIV-specific CTL presented by HLA molecules such as HLA-B8. He argued that a consequence of this variation is a
IMMUNE
SYSTEM
PHYSIOLOGY
broadening of the immune response from the normal immunodominant pattern, focusing on a very small number of epitopes, to broad responses to multiple epitopes. Thus, whereas influenza-specific and EBV-specific responses tend to focus on very few epitopes, sometimes with single dominant receptors, anti-HIV responses are usually broad. However, in a few patients, CTL responses may focus on highly conserved sequences and therefore be restricted in heterogeneity. He suggested that the failure of CTL to control HIV infection may result from the continuous escape of HIV from CTL control, only partly controlled by new T-cell responses. As the infection progresses, CD4 T-cell help declines and the infected patient may have difficulty in
AND
PATHOLOGY
413
generating new CTL responses to variant virus epitopes and in maintaining old CTL responses. Overall, the session highlighted a number of mechanisms of immune escape from CTL control of virus infections. The strategies demonstrate the importance of CTL in controlling virus infections. Often, escape is associated with persistence of the virus. In many cases, the stable equilibrium is reached where the virus is largely controlled, although persisting but not causing harm (e.g. HSV, CMV, EBV). However, in some cases, such an equilibrium is not reached (HIV1 and HIV2). Understanding these virus escape strategies gives some rational basis on which to develop immunotherapies.