Factors associated with slow disease progression in macaques immunized with an adenovirus-simian immunodeficiency virus (SIV) envelope priming-gp120 boosting regimen and challenged vaginally with SIVmac251

Abstract

Rhesus macaques were immunized with a combination vaccine regimen consisting of adenovirus type 5 host range mutant-simian immunodeficiency virus envelope (Ad5hr-SIVenv) recombinant priming and boosting with native SIV gp120. Upon intravaginal challenge with SIVmac251, both persistently and transiently viremic animals were observed (S. L. Buge, E. Richardson, S. Alipanah, P. Markham, S. Cheng, N. Kalyan, C. J. Miller, M. Lubeck, S. Udem, J. Eldridge, and M. Robert-Guroff, J. Virol. 71:8531-8541, 1997). Long-term follow-up of the persistently viremic immunized macaques, which displayed significantly reduced viral burdens during the first 18 weeks postchallenge compared to controls, has now shown that one of four became a slow progressor, clearing virus from plasma and remaining asymptomatic with stable CD4 counts for 134 weeks postchallenge. Reboosting of the transiently viremic macaques did not reactivate latent virus. Rechallenge with two sequential SIVmac251 intravaginal exposures again resulted in partial protection of one of two immunized macaques, manifested by viral clearance and stable CD4 counts. No single immune parameter was associated with partial protection. Development of a strong antibody response capable of neutralizing a primary SIVmac251 isolate together with SIV-specific cytotoxic T lymphocytes were implicated, while CD8(+) T-cell antiviral activity and mucosal immune responses were not associated with delayed disease progression. Our data show that even a third immunization with the same Ad5hr-SIVenv recombinant can elicit significant immune responses to the inserted gene product, suggesting that preexisting Ad antibodies may not preclude effective immunization. Further, the partial protection against a virulent, pathogenic SIV challenge observed in two of six macaques immunized with a vaccine regimen based solely on the viral envelope indicates that this vectored-vaccine approach has promise and that multicomponent vaccines based in the same system merit further investigation.

FIG. 1

Immunization and challenge protocol. Twelve macaques, six immunized (no. 353 to 358) and six controls (no. 359 to 364), were immunized and challenged according to the schedule shown at the top of the figure (in weeks) as reported previously (8). Subsequently the five macaques indicated were reboosted and rechallenged as described at the bottom of the figure. Macaque 359 did not become infected following the second challenge and was not included further in this study.

FIG. 2

Long-term follow-up of persistently viremic macaques following the first vaginal challenge with SIVmac251. The death of a macaque is indicated by a dagger symbol. The lower limit of detection of RNA by the NASBA technique is indicated by the dashed line. After its CD4-cell count had been approximately 100 for over 50 weeks, macaque 360 was treated with a peptide polymer modeling the CD4 binding site on SIV gp120 on four occasions (weeks 113, 117, 123, and 128) with no apparent effect on CD4-cell number or viral load (51a). Macaque 356 was similarly treated once at week 113 when its CD4-cell count was already below 100. It was euthanized at week 116.

FIG. 3

Neutralization of lab-adapted SIVmac251 by macaque sera after the first vaginal challenge. Neutralizing-antibody assays were carried out over 10 weeks postchallenge as described in Materials and Methods.

FIG. 4

Pepscan analysis of macaque sera, using SIVmac251 gp120. Sera of persistently viremic immunized macaques were subjected to an ELISA at the time of the first challenge (19 March 1996), and transiently viremic immunized macaques were assayed at the time of the rechallenge (2 July 1997).

FIG. 4

Pepscan analysis of macaque sera, using SIVmac251 gp120. Sera of persistently viremic immunized macaques were subjected to an ELISA at the time of the first challenge (19 March 1996), and transiently viremic immunized macaques were assayed at the time of the rechallenge (2 July 1997).

FIG. 5

Outcome of second vaginal challenge of macaques transiently viremic following the first challenge. Results of virus isolation attempts (VI) following the rechallenge are shown (+, virus isolation positive; −, virus isolation negative). Control macaques 364 and 361 were also treated with the peptide polymer modeling the CD4 binding site on SIV gp120, as outlined in the legend to Fig. 2, on three and four occasions, respectively (weeks 46, 50, 56, and 61), with no apparent benefit. The macaques subsequently were euthanized at weeks 57 and 73, respectively.

FIG. 6

CTL activity after reboost and at the time of the second vaginal challenge. Assays were carried out on viably frozen lymphocytes. vac, vaccinia virus recombinant containing the indicated gene insert.

FIG. 7

CD8⁺ T-cell antiviral activity pre- and post-second challenge. Assays were carried out on viably frozen lymphocytes. Suppression by macaque 361 PBMCs could not be determined because control levels of infection in the endogenous assay were too low. Additional PBMCs of macaque 361 were not available for reassay.

FIG. 8

Antibody reactivity of macaque sera to SIV gp120 in secretory fluids at the time of the reboost and rechallenge. No binding antibodies were detected in vaginal fluids.