- Email: [email protected]
Live attenuated SIV—a model of a vaccine for AIDS
Immunology Letters 66 (1999) 167 – 170
Live attenuated SIV —a model of a vaccine for AIDS Neil Almond *, Jim Stott Di6ision of Retro6irology, National Institute for Biological Standards & Control (NIBSC), South Mimms, Potters Bar, Herts. ENO 3QG, UK Received 19 October 1998; accepted 18 November 1998
Abstract The experimental infection of macaques with simian immunodeficiency virus (SIV) has provided strong evidence that it may be possible to develop a vaccine against AIDS. Live attenuated SIV vaccines have been found to confer the most potent protection against challenge with a variety of pathogenic viruses. This article summarizes the work performed at NIBSC to characterize the protection conferred by live attenuated SIV and to identify mechanisms of vaccine protection. The results of these experiments are discussed in conjunction with observations from related studies made by other groups. © 1999 Elsevier Science B.V. All rights reserved. Keywords: AIDS; Vaccines; SIV; Attenuation
1. Introduction The development of an effective vaccine remains the best approach to control the spread of HIV and AIDS. The announcement last year that a group of physicians had volunteered to take live attenuated HIV-1, in order to initiate human safety studies [1], focused attention on this type of vaccine and the positive results which they have generated in animal models. However, the subsequent debate highlighted our limited knowledge about live attenuated AIDS vaccines: the mechanism of protection, correlates of vaccine protection and critical issues relating to the safe use of live retroviruses as vaccines in humans. These exciting studies began in l991, when the group of Ron Desrosiers at the New England Primate Research Center published observations that the irreversible genetic disruption of the nef gene of the pathogenic clone SIVmac239 attenuated the course of infection in rhesus macaques. In contrast to macaques receiving the wild-type clone, a group of six macaques
* Corresponding author. Tel.: +44-1707-654753; fax: +44-1707649865; e-mail: [email protected].
inoculated with the disrupted clone SIVmac239Dnef remained clinically healthy for more than 3 years despite the persistence of the virus [2]. More exciting was the observation that when these macaques were rechallenged intravenously with wild-type uncloned SIVmac251, there was no evidence for superinfection [3]. We have confirmed the potency of this approach using an independently produced attenuated clone of SIVmac32H called C8 [4]. Over the past 6 years using this C8 vaccine, we have repeatedly protected macaques against detectable superinfection with wild-type SIV presented as cell-free virus and virus infected cells [5] (Table 1). Others have also demonstrated that this vaccine protects against rectal challenge with SIVmac [6]. Thus in this model system, live attenuated SIV fulfil many of the criteria expected of an effective AIDS vaccine and these results led to the calls to test this vaccine approach in humans. A number of safety issues have tempered the rush to use attenuated HIV as a prophylactic vaccine in humans. In addition to the danger that a live retrovirus could initiate cancer through insertional mutagenesis of the provirus in the host cell genome, there is concern that the widespread use of this type of vaccine may
0165-2478/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 4 7 8 ( 9 8 ) 0 0 1 5 3 - 9
N. Almond, J. Stott / Immunology Letters 66 (1999) 167–170
168
Table 1 Summary of vaccine protection obtained in studies at NIBSC using live attenuated SIV followed with wild-type SIV (1992–1997) Vaccine Virus
Challenge virus and outcomea J5b J82c
SIVDnef (C8 and related viruses) None
0/36 20/20
a b
0/4 4/4
Number of macaques infected/number of macaques challenged. Cell-free SIV challenge stocks prepared on macaque lymphocytes
(5). c
SIV infected spleen cells derived from a macaque 10 weeks after challenge with SIVmac (5).
cause AIDS or neurological disease in selected groups of patients. Baba and co-workers [7] reported that clones of SIVmac which expressed an attenuated phenotype in adult rhesus macaques due to three distinct genetic lesions were capable of causing AIDS when administered orally to neonates. In addition, a number of groups have reported that a proportion of healthy adult macaques inoculated with attenuated SIV are unable to control the infection and progress onto disease [8]. Although during this process repair of attenuating genetic disruptions have been observed [9,10], high level virus replication precedes detection of the revertant viruses. This would indicate that host factors may be a part of the failure of selected individuals to control infection with attenuated SIV.
2. Characterisation of vaccine protection Safety concerns over the use of live attenuated HIV as a prophylactic AIDS vaccine may take many years to resolve. Attenuated SIV induces by far the most potent protection observed to date. If we are able to unravel the mechanism of this potent vaccine protection then it may be possible to reproduce it by a less Table 2 Summary of vaccine protection obtained in studies at NIBSC using live attenuated SIV followed by challenge with SIV/HIV chimeras or SHIV’s Vaccine Virus
SIVDnef SIVJ5 None a
Challenge virus and outcomea SHIV-4b
SHIVSF33c
SH1V89.6Pd
0/4 0/4 3/4
0/4 0/4 4/4
3/4 1/4 4/4
Number of macaques infected/number of macaques challenged. Ref. [16]. c Ref. [19]. d Ref. [20]. b
hazardous means. A series of experiments have been carried out at NIBSC to elucidate questions regarding the mechanism by which these vaccines protect against superinfection.
2.1. Role of antibody Immune serum was collected from four cynomolgus macaques which had been infected with SIVmacC8 for 104 weeks and had resisted challenge with wild-type SIVmacJ5 at 39 weeks. Two naive macaques were injected intraperitoneally with unheated pooled immune serum (11 ml/kg body weight) and, 24 h later challenged with cell free SIVmacJ5, together with four untreated naive controls. Although antibodies were successfully transferred and detectable in the circulation of the two recipient macaques on the day of virus challenge, all six macaques became infected and cell associated virus loads were indistinguishable between the two groups [11]. Thus, immune serum was not able to transfer this vaccine protection.
2.2. Role of cell mediated immunity As macaques are outbred, it is not possible to perform adoptive transfer of immune lymphocytes from immunised to naive macaques. We have used antibody mediated lymphocyte depletion of C8 infected macaques prior to re-challenge, to determine the role that cell mediated immune responses and, in particular, cytotoxic T-cell responses play in this vaccine protection. By infusion of a cocktail of two rat anti-human CD8 monoclonal antibodies, it was possible to deplete CD2+ CD4− CD8+ cells in 2 of 4 C8 immunised macaques by greater than 90% in the periphery and by approximately 50% in lymph nodes on the day of challenge with wild-type SIVmacJ5. This level of depletion was similar to that obtained by Matano and co-workers [12], and in that study it was sufficient to exacerbate the primary viraemia of an SIV/HlV-1 chimera in naive macaques. However, in our study none of the four C8 immunised anti-CD8 treated macaques became infected with SIVmacJ5 upon challenge [13]. In a subsequent experiment, a further four C8 infected macaques were treated with CAMPATH1H, a humanised rat anti-human CD52 monoclonal. This successfully depleted greater than 99% of CD3+ cells from the periphery in all four macaques and two naive controls treated similarly. Nevertheless all four control macaques became infected with SIVmacJ5 upon challenge and none of the C8 infected, CAMPATH-IH treated macaques became superinfected. Thus partial depletion of CD8+ or a more profound suppression of CD52+ lymphocytes failed to abrogate the protection conferred by the C8 vaccine [13].
N. Almond, J. Stott / Immunology Letters 66 (1999) 167–170
2.3. Viral target of 6accine protection An effective AIDS vaccine will have to protect against challenge with HIV-1 isolates which express a diverse range of envelope proteins. We and others have demonstrated that immunisation with attenuated SIV can protect against detectable superinfection following intravenous [14,15] and intrarectal [6] challenge with the chimeric virus SHIV-4 [16] which expresses the tat, re6 and en6 genes of the genetically and antigenically distinct HIV-1IIIB isolate. However this chimera replicates relatively poorly in vivo due to the disruption of the 6pu gene and thus these results may provide an overestimate of the vaccine potency. Others support this view and have shown that re-challenge with more vigorous SHIV’s can lead to break-through although virus loads are reduced [17,18]. We too have evaluated the protection conferred by chronic infection with SIVmacC8 against challenge with more vigorous SHIV’s (SHIVsf33 [19], SHIV89.6P [20], Table 2). Following challenge with SHIVsf33, there was no evidence of superinfection either from repeated sampling of the peripheral blood or careful examination of a variety of lymphoid tissues post mortem. Although there was a marked vaccine effect following challenge with SHlV89.6P, this virus was detected in the blood of only one out of four C8 immunised macaques, at post mortem the superinfecting virus was detected in the lymphoid tissue of three of the same macaques. Interestingly, similar results (Table 2), that is, complete protection against challenge with SHIVsf33 and partial protection against challenge with SHlV89.6P, have been obtained using cynomolgus macaques chronically infected with the wild-type virus SIVmacJ5, indicating that the protection observed is not a special property of infection with an attenuated virus.
2.4. Time to 6accine protection The potent protection against superinfection induced by chronic infection with SIVmacC8 has resulted in few unprotected, vaccinated macaques. This, in turn, has made it difficult to identify correlates of vaccine protection by comparing protected with unprotected vaccinated animals. As an alternative approach to this question, a number of groups have set out to determine the length of time from immunisation with attenuated virus before resistance to superinfection is obtained. In contrast to the report by Wyand et al. [21] suggesting that as much as 79 weeks of infection with SIVmacD3 was needed before solid protection could be obtained, Norley and his co-workers reported that three out of four rhesus macaques immunised with SIVmacC8 were protected against re-challenge by 10 weeks [22]. We have recently extended these studies evaluating the proportion of cynomolgus macaques protected against
169
challenge with wild-type J5 after a 10, 6 or 3 week period of immunisation. Surprisingly, only three weeks were required to induce protection against detectable superinfection with SIVmacJ5 at a time when no neutralising activity or ELISA antibodies to SIV Gag or Envelope were detectable in the blood.
3. Discussion Although our experiments have not yet provided a definitive answer to our question as to the mechanism of protection conferred by live attenuated SIVmac vaccines, the results obtained provide a striking contrast to most successful viral vaccines and some intriguing clues. The combined evidence from the passive transfer of immune serum, challenge with SHIV chimeras and studies of the time to protection would indicate that it is unlikely that antibody alone mediates vaccine protection. The results from other groups are contradictory. Preliminary results from Wyand et al [23] using the SIVmac239D3 vaccine supports our conclusions, whereas Clements et al [24] using immune globulin transferred from macaques infected with a macrophage tropic virus vaccine protected two out of four recipients. Nevertheless, since there is little antigenic similarity between SIV and HIV-1 envelope and no cross neutralising activity between the two viruses [17,18], it is unlikely that this would account for the protection observed against challenge with SIV-HIV-1 envelope chimeras. The data regarding the role of CD8+ cells in protection is less clear cut. Although attempts to deplete CD8+ lymphocytes were incomplete, others have demonstrated that this level of depletion is sufficient to alter the course of the primary viraemia [12]. Furthermore, CD8+ CTL responses have been detected within 3 weeks of infection with attenuated viruses [25]. Nevertheless, it should not be forgotten that anti-HIV activity has been described for CD8 + cells through mechanisms in addition to classical T-cell cytotoxicity [26] and this has been implicated in the control of infection by baboons with HIV-2 vaccines [27]. Similar activity has been detected in PBMC of cynomolgus macaques following immunisation with SIVmacC8 [28], but neither the magnitude nor the onset of its expression is sufficient alone to account for the vaccine protection. If immune mechanisms do not provide a suitable correlate then what alternative mechanisms may be involved in vaccine protection? Retroviral interference has described in other systems [29]. At first sight, it is difficult to envisage that direct interference by receptor competition is likely in the protection conferred by attenuated SIV in vivo, since only a small proportion of CD4+ cells are infected. However, the recent report by Feazey et al [30] indicating that gut associated lymphoid tissue provides the focus of early SIV infec-
170
N. Almond, J. Stott / Immunology Letters 66 (1999) 167–170
tion suggests that interference by blocking only a proportion of CD4 + cells in specific locations may confer a more effective vaccine effect than otherwise expected. If this is the case, that local rather than systemic protection is required, then new experiments will need to be designed to address this possibility. The experimental infection of macaques with SIV has provided evidence that solid vaccine protection against HIV infection of humans is feasible. To turn these observations into a practical AIDS vaccine for man, will require further carefully designed experiments in the macaque model to elucidate further the precise mechanism.
Acknowledgements This work was supported in part by the UK Medical Research Council (Grant Number G9025730) and EU Biomed Programme (Grant Number BMH1-CT931795). The authors would like to thank Barry Walker, Richard Stebbings, Alison Wade-Evans and Robin Hull at NIBSC and Martin Cranage at CAMR, Porton Down for valuable discussions through the course of this work, and Lynn Bays for secretarial assistance.
References [1] J. Cohen, Science 277 (1997) 1035. [2] H.D. Kestler, D.J. Ringler, K. Mori, et al., Cell 65 (1992) 651 – 662. [3] M.D. Daniel, F. Kirchoff, S.C. Czajak, P.K. Sehgal, R.C. Desrosier, Science 258 (1992) 1938–1941. [4] E.W. Rud, M. Cranage, J. Yon, et al., J. Gen. Virol. 75 (1994) 529 – 543. [5] N. Almond, K. Kent, M. Cranage, E. Rud, B. Clarke, E.J. Stott, Lancet 345 (1995) 1342–1344. [6] M.P. Cranage, A.M. Whatmore, S.A. Sharpe, et al., Virology 229 (1997) 143 – 154.
.
[7] T.W. Baba, Y.S. Jeony, D. Pennick, R. Bronson, M.F. Greene, R.M. Ruprecht, Science 267 (1995) 1820 – 1825. [8] J. Cohen, Science 278 (1997) 24 – 25. [9] A. Whatmore, N. Cook, G.A. Hall, S. Sharpe, E. Rud, M.P. Cranage, J. Virol. 69 (1995) 5117 – 5123. [10] C. Stahl-Hennig, U. Dittmer, T. Nißlein, et al., J. Gen. Virol. 77 (1996) 2969 – 2981. [11] N. Almond, J. Rose, R. Sangster, et al., J. Gen. Virol. 78 (1997) 1919 – 1922. [12] T. Matano, R. Shibata, C. Siemon, et al., J. Virol. 72 (1998) 164 – 169. [13] R. Stebbings, E.J. Stott, N. Almond, et al., AIDS. Hum. Retro 14 (1998) 1187 – 1198. [14] E.J. Stott, N. Almond, W. West, K. Kent, M. Cranage, E. Rud, in: M. Girard (Ed.), Neuvieme Colloques des Cents Gardes, Fondation Meriuex, Lyon, 1994, pp. 219 – 224. [15] W.M. Bogers, H. Niphuis, P. ten-Haaft, J.D. Laman, W. Koornstra, J.L. Heeney, AIDS 9 (1995) F13 – F18. [16] J. Li, C.I. Lord, W. Haseltine, N.L. Letvin, J. Sodroski, J. AIDS 5 (1992) 639 – 646. [17] C.S. Dunn, B. Hurtrel, C. Beyer, et al., AIDS Res. Hum. Retro 13 (1997) 913 – 922. [18] R. Shibata, C. Siemon, S.C. Czajak, R.C. Desrosiers, M.A. Martin, J. Virol. 71 (1997) 8141 – 8148. [19] P. Luciw, E. Pratt-Lowe, K. Shaw, J. Levy, C. Cheng-Meyer, Proc. Natl. Acad. Sci., USA 92 (1995) 7490 – 7494. [20] K. Reimann, J. Li, R. Veazey, et al., J. Virol. 70 (1996) 6922– 6928. [21] M. Wyand, K. Manson, M. Garcia-Moll, D. Montefiori, R.C. Desrosiers, J. Virol. 70 (1996) 3722 – 3724. [22] S. Norley, B. Beer, D. Binninger-Schinzel, C. Cosma, R. Kurth, Virology 219 (1996) 195 – 205. [23] N.L. Letvin, Science 280 (1998) 1875 – 1880. [24] J. Clements, R. Montelaro, C. Zink, et al., J. Virol. 69 (1995) 2737 – 2744. [25] R.P. Johnson, R.L. Glickman, J.Q. Yang, et al., J. Virol. 71 (1997) 7711 – 7718. [26] J.A. Levy, C.E. Mackewicz, E. Barker, Immunol. Today 17 (1996) 217 – 224. [27] C. Locher, D. Blackbown, S. Barnett, et al., J. Inf. Dis. 176 (1997) 948 – 959. [28] P. Silvera, B. Flannagan, K. Kent, et al., AIDS Res. Hum. Retro. 10 (1994) 1295 – 1304. [29] T. Mitchell, R. Risser, J. Virol. 66 (1992) 5696 – 5702. [30] R. Veazey, M. De Maria, L. Chalifoux, et al., Science 280 (1998) 427 – 431.
.
Live attenuated SIV —a model of a vaccine for AIDS Neil Almond *, Jim Stott Di6ision of Retro6irology, National Institute for Biological Standards & Control (NIBSC), South Mimms, Potters Bar, Herts. ENO 3QG, UK Received 19 October 1998; accepted 18 November 1998
Abstract The experimental infection of macaques with simian immunodeficiency virus (SIV) has provided strong evidence that it may be possible to develop a vaccine against AIDS. Live attenuated SIV vaccines have been found to confer the most potent protection against challenge with a variety of pathogenic viruses. This article summarizes the work performed at NIBSC to characterize the protection conferred by live attenuated SIV and to identify mechanisms of vaccine protection. The results of these experiments are discussed in conjunction with observations from related studies made by other groups. © 1999 Elsevier Science B.V. All rights reserved. Keywords: AIDS; Vaccines; SIV; Attenuation
1. Introduction The development of an effective vaccine remains the best approach to control the spread of HIV and AIDS. The announcement last year that a group of physicians had volunteered to take live attenuated HIV-1, in order to initiate human safety studies [1], focused attention on this type of vaccine and the positive results which they have generated in animal models. However, the subsequent debate highlighted our limited knowledge about live attenuated AIDS vaccines: the mechanism of protection, correlates of vaccine protection and critical issues relating to the safe use of live retroviruses as vaccines in humans. These exciting studies began in l991, when the group of Ron Desrosiers at the New England Primate Research Center published observations that the irreversible genetic disruption of the nef gene of the pathogenic clone SIVmac239 attenuated the course of infection in rhesus macaques. In contrast to macaques receiving the wild-type clone, a group of six macaques
* Corresponding author. Tel.: +44-1707-654753; fax: +44-1707649865; e-mail: [email protected].
inoculated with the disrupted clone SIVmac239Dnef remained clinically healthy for more than 3 years despite the persistence of the virus [2]. More exciting was the observation that when these macaques were rechallenged intravenously with wild-type uncloned SIVmac251, there was no evidence for superinfection [3]. We have confirmed the potency of this approach using an independently produced attenuated clone of SIVmac32H called C8 [4]. Over the past 6 years using this C8 vaccine, we have repeatedly protected macaques against detectable superinfection with wild-type SIV presented as cell-free virus and virus infected cells [5] (Table 1). Others have also demonstrated that this vaccine protects against rectal challenge with SIVmac [6]. Thus in this model system, live attenuated SIV fulfil many of the criteria expected of an effective AIDS vaccine and these results led to the calls to test this vaccine approach in humans. A number of safety issues have tempered the rush to use attenuated HIV as a prophylactic vaccine in humans. In addition to the danger that a live retrovirus could initiate cancer through insertional mutagenesis of the provirus in the host cell genome, there is concern that the widespread use of this type of vaccine may
0165-2478/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 4 7 8 ( 9 8 ) 0 0 1 5 3 - 9
N. Almond, J. Stott / Immunology Letters 66 (1999) 167–170
168
Table 1 Summary of vaccine protection obtained in studies at NIBSC using live attenuated SIV followed with wild-type SIV (1992–1997) Vaccine Virus
Challenge virus and outcomea J5b J82c
SIVDnef (C8 and related viruses) None
0/36 20/20
a b
0/4 4/4
Number of macaques infected/number of macaques challenged. Cell-free SIV challenge stocks prepared on macaque lymphocytes
(5). c
SIV infected spleen cells derived from a macaque 10 weeks after challenge with SIVmac (5).
cause AIDS or neurological disease in selected groups of patients. Baba and co-workers [7] reported that clones of SIVmac which expressed an attenuated phenotype in adult rhesus macaques due to three distinct genetic lesions were capable of causing AIDS when administered orally to neonates. In addition, a number of groups have reported that a proportion of healthy adult macaques inoculated with attenuated SIV are unable to control the infection and progress onto disease [8]. Although during this process repair of attenuating genetic disruptions have been observed [9,10], high level virus replication precedes detection of the revertant viruses. This would indicate that host factors may be a part of the failure of selected individuals to control infection with attenuated SIV.
2. Characterisation of vaccine protection Safety concerns over the use of live attenuated HIV as a prophylactic AIDS vaccine may take many years to resolve. Attenuated SIV induces by far the most potent protection observed to date. If we are able to unravel the mechanism of this potent vaccine protection then it may be possible to reproduce it by a less Table 2 Summary of vaccine protection obtained in studies at NIBSC using live attenuated SIV followed by challenge with SIV/HIV chimeras or SHIV’s Vaccine Virus
SIVDnef SIVJ5 None a
Challenge virus and outcomea SHIV-4b
SHIVSF33c
SH1V89.6Pd
0/4 0/4 3/4
0/4 0/4 4/4
3/4 1/4 4/4
Number of macaques infected/number of macaques challenged. Ref. [16]. c Ref. [19]. d Ref. [20]. b
hazardous means. A series of experiments have been carried out at NIBSC to elucidate questions regarding the mechanism by which these vaccines protect against superinfection.
2.1. Role of antibody Immune serum was collected from four cynomolgus macaques which had been infected with SIVmacC8 for 104 weeks and had resisted challenge with wild-type SIVmacJ5 at 39 weeks. Two naive macaques were injected intraperitoneally with unheated pooled immune serum (11 ml/kg body weight) and, 24 h later challenged with cell free SIVmacJ5, together with four untreated naive controls. Although antibodies were successfully transferred and detectable in the circulation of the two recipient macaques on the day of virus challenge, all six macaques became infected and cell associated virus loads were indistinguishable between the two groups [11]. Thus, immune serum was not able to transfer this vaccine protection.
2.2. Role of cell mediated immunity As macaques are outbred, it is not possible to perform adoptive transfer of immune lymphocytes from immunised to naive macaques. We have used antibody mediated lymphocyte depletion of C8 infected macaques prior to re-challenge, to determine the role that cell mediated immune responses and, in particular, cytotoxic T-cell responses play in this vaccine protection. By infusion of a cocktail of two rat anti-human CD8 monoclonal antibodies, it was possible to deplete CD2+ CD4− CD8+ cells in 2 of 4 C8 immunised macaques by greater than 90% in the periphery and by approximately 50% in lymph nodes on the day of challenge with wild-type SIVmacJ5. This level of depletion was similar to that obtained by Matano and co-workers [12], and in that study it was sufficient to exacerbate the primary viraemia of an SIV/HlV-1 chimera in naive macaques. However, in our study none of the four C8 immunised anti-CD8 treated macaques became infected with SIVmacJ5 upon challenge [13]. In a subsequent experiment, a further four C8 infected macaques were treated with CAMPATH1H, a humanised rat anti-human CD52 monoclonal. This successfully depleted greater than 99% of CD3+ cells from the periphery in all four macaques and two naive controls treated similarly. Nevertheless all four control macaques became infected with SIVmacJ5 upon challenge and none of the C8 infected, CAMPATH-IH treated macaques became superinfected. Thus partial depletion of CD8+ or a more profound suppression of CD52+ lymphocytes failed to abrogate the protection conferred by the C8 vaccine [13].
N. Almond, J. Stott / Immunology Letters 66 (1999) 167–170
2.3. Viral target of 6accine protection An effective AIDS vaccine will have to protect against challenge with HIV-1 isolates which express a diverse range of envelope proteins. We and others have demonstrated that immunisation with attenuated SIV can protect against detectable superinfection following intravenous [14,15] and intrarectal [6] challenge with the chimeric virus SHIV-4 [16] which expresses the tat, re6 and en6 genes of the genetically and antigenically distinct HIV-1IIIB isolate. However this chimera replicates relatively poorly in vivo due to the disruption of the 6pu gene and thus these results may provide an overestimate of the vaccine potency. Others support this view and have shown that re-challenge with more vigorous SHIV’s can lead to break-through although virus loads are reduced [17,18]. We too have evaluated the protection conferred by chronic infection with SIVmacC8 against challenge with more vigorous SHIV’s (SHIVsf33 [19], SHIV89.6P [20], Table 2). Following challenge with SHIVsf33, there was no evidence of superinfection either from repeated sampling of the peripheral blood or careful examination of a variety of lymphoid tissues post mortem. Although there was a marked vaccine effect following challenge with SHlV89.6P, this virus was detected in the blood of only one out of four C8 immunised macaques, at post mortem the superinfecting virus was detected in the lymphoid tissue of three of the same macaques. Interestingly, similar results (Table 2), that is, complete protection against challenge with SHIVsf33 and partial protection against challenge with SHlV89.6P, have been obtained using cynomolgus macaques chronically infected with the wild-type virus SIVmacJ5, indicating that the protection observed is not a special property of infection with an attenuated virus.
2.4. Time to 6accine protection The potent protection against superinfection induced by chronic infection with SIVmacC8 has resulted in few unprotected, vaccinated macaques. This, in turn, has made it difficult to identify correlates of vaccine protection by comparing protected with unprotected vaccinated animals. As an alternative approach to this question, a number of groups have set out to determine the length of time from immunisation with attenuated virus before resistance to superinfection is obtained. In contrast to the report by Wyand et al. [21] suggesting that as much as 79 weeks of infection with SIVmacD3 was needed before solid protection could be obtained, Norley and his co-workers reported that three out of four rhesus macaques immunised with SIVmacC8 were protected against re-challenge by 10 weeks [22]. We have recently extended these studies evaluating the proportion of cynomolgus macaques protected against
169
challenge with wild-type J5 after a 10, 6 or 3 week period of immunisation. Surprisingly, only three weeks were required to induce protection against detectable superinfection with SIVmacJ5 at a time when no neutralising activity or ELISA antibodies to SIV Gag or Envelope were detectable in the blood.
3. Discussion Although our experiments have not yet provided a definitive answer to our question as to the mechanism of protection conferred by live attenuated SIVmac vaccines, the results obtained provide a striking contrast to most successful viral vaccines and some intriguing clues. The combined evidence from the passive transfer of immune serum, challenge with SHIV chimeras and studies of the time to protection would indicate that it is unlikely that antibody alone mediates vaccine protection. The results from other groups are contradictory. Preliminary results from Wyand et al [23] using the SIVmac239D3 vaccine supports our conclusions, whereas Clements et al [24] using immune globulin transferred from macaques infected with a macrophage tropic virus vaccine protected two out of four recipients. Nevertheless, since there is little antigenic similarity between SIV and HIV-1 envelope and no cross neutralising activity between the two viruses [17,18], it is unlikely that this would account for the protection observed against challenge with SIV-HIV-1 envelope chimeras. The data regarding the role of CD8+ cells in protection is less clear cut. Although attempts to deplete CD8+ lymphocytes were incomplete, others have demonstrated that this level of depletion is sufficient to alter the course of the primary viraemia [12]. Furthermore, CD8+ CTL responses have been detected within 3 weeks of infection with attenuated viruses [25]. Nevertheless, it should not be forgotten that anti-HIV activity has been described for CD8 + cells through mechanisms in addition to classical T-cell cytotoxicity [26] and this has been implicated in the control of infection by baboons with HIV-2 vaccines [27]. Similar activity has been detected in PBMC of cynomolgus macaques following immunisation with SIVmacC8 [28], but neither the magnitude nor the onset of its expression is sufficient alone to account for the vaccine protection. If immune mechanisms do not provide a suitable correlate then what alternative mechanisms may be involved in vaccine protection? Retroviral interference has described in other systems [29]. At first sight, it is difficult to envisage that direct interference by receptor competition is likely in the protection conferred by attenuated SIV in vivo, since only a small proportion of CD4+ cells are infected. However, the recent report by Feazey et al [30] indicating that gut associated lymphoid tissue provides the focus of early SIV infec-
170
N. Almond, J. Stott / Immunology Letters 66 (1999) 167–170
tion suggests that interference by blocking only a proportion of CD4 + cells in specific locations may confer a more effective vaccine effect than otherwise expected. If this is the case, that local rather than systemic protection is required, then new experiments will need to be designed to address this possibility. The experimental infection of macaques with SIV has provided evidence that solid vaccine protection against HIV infection of humans is feasible. To turn these observations into a practical AIDS vaccine for man, will require further carefully designed experiments in the macaque model to elucidate further the precise mechanism.
Acknowledgements This work was supported in part by the UK Medical Research Council (Grant Number G9025730) and EU Biomed Programme (Grant Number BMH1-CT931795). The authors would like to thank Barry Walker, Richard Stebbings, Alison Wade-Evans and Robin Hull at NIBSC and Martin Cranage at CAMR, Porton Down for valuable discussions through the course of this work, and Lynn Bays for secretarial assistance.
References [1] J. Cohen, Science 277 (1997) 1035. [2] H.D. Kestler, D.J. Ringler, K. Mori, et al., Cell 65 (1992) 651 – 662. [3] M.D. Daniel, F. Kirchoff, S.C. Czajak, P.K. Sehgal, R.C. Desrosier, Science 258 (1992) 1938–1941. [4] E.W. Rud, M. Cranage, J. Yon, et al., J. Gen. Virol. 75 (1994) 529 – 543. [5] N. Almond, K. Kent, M. Cranage, E. Rud, B. Clarke, E.J. Stott, Lancet 345 (1995) 1342–1344. [6] M.P. Cranage, A.M. Whatmore, S.A. Sharpe, et al., Virology 229 (1997) 143 – 154.
.
[7] T.W. Baba, Y.S. Jeony, D. Pennick, R. Bronson, M.F. Greene, R.M. Ruprecht, Science 267 (1995) 1820 – 1825. [8] J. Cohen, Science 278 (1997) 24 – 25. [9] A. Whatmore, N. Cook, G.A. Hall, S. Sharpe, E. Rud, M.P. Cranage, J. Virol. 69 (1995) 5117 – 5123. [10] C. Stahl-Hennig, U. Dittmer, T. Nißlein, et al., J. Gen. Virol. 77 (1996) 2969 – 2981. [11] N. Almond, J. Rose, R. Sangster, et al., J. Gen. Virol. 78 (1997) 1919 – 1922. [12] T. Matano, R. Shibata, C. Siemon, et al., J. Virol. 72 (1998) 164 – 169. [13] R. Stebbings, E.J. Stott, N. Almond, et al., AIDS. Hum. Retro 14 (1998) 1187 – 1198. [14] E.J. Stott, N. Almond, W. West, K. Kent, M. Cranage, E. Rud, in: M. Girard (Ed.), Neuvieme Colloques des Cents Gardes, Fondation Meriuex, Lyon, 1994, pp. 219 – 224. [15] W.M. Bogers, H. Niphuis, P. ten-Haaft, J.D. Laman, W. Koornstra, J.L. Heeney, AIDS 9 (1995) F13 – F18. [16] J. Li, C.I. Lord, W. Haseltine, N.L. Letvin, J. Sodroski, J. AIDS 5 (1992) 639 – 646. [17] C.S. Dunn, B. Hurtrel, C. Beyer, et al., AIDS Res. Hum. Retro 13 (1997) 913 – 922. [18] R. Shibata, C. Siemon, S.C. Czajak, R.C. Desrosiers, M.A. Martin, J. Virol. 71 (1997) 8141 – 8148. [19] P. Luciw, E. Pratt-Lowe, K. Shaw, J. Levy, C. Cheng-Meyer, Proc. Natl. Acad. Sci., USA 92 (1995) 7490 – 7494. [20] K. Reimann, J. Li, R. Veazey, et al., J. Virol. 70 (1996) 6922– 6928. [21] M. Wyand, K. Manson, M. Garcia-Moll, D. Montefiori, R.C. Desrosiers, J. Virol. 70 (1996) 3722 – 3724. [22] S. Norley, B. Beer, D. Binninger-Schinzel, C. Cosma, R. Kurth, Virology 219 (1996) 195 – 205. [23] N.L. Letvin, Science 280 (1998) 1875 – 1880. [24] J. Clements, R. Montelaro, C. Zink, et al., J. Virol. 69 (1995) 2737 – 2744. [25] R.P. Johnson, R.L. Glickman, J.Q. Yang, et al., J. Virol. 71 (1997) 7711 – 7718. [26] J.A. Levy, C.E. Mackewicz, E. Barker, Immunol. Today 17 (1996) 217 – 224. [27] C. Locher, D. Blackbown, S. Barnett, et al., J. Inf. Dis. 176 (1997) 948 – 959. [28] P. Silvera, B. Flannagan, K. Kent, et al., AIDS Res. Hum. Retro. 10 (1994) 1295 – 1304. [29] T. Mitchell, R. Risser, J. Virol. 66 (1992) 5696 – 5702. [30] R. Veazey, M. De Maria, L. Chalifoux, et al., Science 280 (1998) 427 – 431.
.