Protection by live, attenuated simian immunodeficiency virus against heterologous challenge

Abstract

We examined the ability of a live, attenuated deletion mutant of simian immunodeficiency virus (SIV), SIVmac239Delta3, which is missing nef and vpr genes, to protect against challenge by heterologous strains SHIV89.6p and SIVsmE660. SHIV89.6p is a pathogenic, recombinant SIV in which the envelope gene has been replaced by a human immunodeficiency virus type 1 envelope gene; other structural genes of SHIV89.6p are derived from SIVmac239. SIVsmE660 is an uncloned, pathogenic, independent isolate from the same primate lentivirus subgrouping as SIVmac but with natural sequence variation in all structural genes. The challenge with SHIV89.6p was performed by the intravenous route 37 months after the time of vaccination. By the criteria of CD4(+) cell counts and disease, strong protection against the SHIV89.6p challenge was observed in four of four vaccinated monkeys despite the complete mismatch of env sequences. However, SHIV89.6p infection was established in all four previously vaccinated monkeys and three of the four developed fluctuating viral loads between 300 and 10,000 RNA copy equivalents per ml of plasma 30 to 72 weeks postchallenge. When other vaccinated monkeys were challenged with SIVsmE660 at 28 months after the time of vaccination, SIV loads were lower than those observed in unvaccinated controls but the level of protection was less than what was observed against SHIV89.6p in these experiments and considerably less than the level of protection against SIVmac251 observed in previous experiments. These results demonstrate a variable level of vaccine protection by live, attenuated SIVmac239Delta3 against heterologous virus challenge and suggest that even live, attenuated vaccine approaches for AIDS will face significant hurdles in providing protection against the natural variation present in field strains of virus. The results further suggest that factors other than anti-Env immune responses can be principally responsible for the vaccine protection by live, attenuated SIV.

FIG. 1

Vaccine phase. (A) Cell-associated vaccine virus loads following vaccination in monkeys that were subsequently challenged with SHIV89.6p. (B) Cell-associated vaccine virus loads following vaccination in monkeys that were subsequently challenged with SIVsmE660. Code for PBMC load: 0, virus was not recovered with 10⁶ or fewer PBMC; 1, virus was recovered with 10⁶ but not fewer PBMC; 2, 333,333 PBMC; 3, 111,111 PBMC; 4, 37,037 PBMC; 5, 12,345 PBMC; 6, 4,115 PBMC; 7, 1,371 PBMC; 8, 457 PBMC. (C) Anti-SIV antibody responses as detected by ELISA following vaccination in monkeys that were subsequently challenged with SHIV89.6p. (D) Anti-SIV antibody responses as detected by ELISA following vaccination in monkeys that were subsequently challenged with SIVsmE660. A410, absorbance at 40 nm.

FIG. 2

Outcome of challenge with SHIV89.6p. (A) CD4⁺ T lymphocytes. (B) Cell-associated virus loads measured as the number of infectious cells in PBMC. Code is as used previously (9, 36): 2, 333,333 PBMC; 3, 111,111 PBMC; 4, 37,037 PBMC; 5, 12,345 PBMC; 6, 4,115 PBMC; 7, 1,371 PBMC; 8, 457 PBMC. (C) Viral RNA levels in plasma. Open symbols used for control animals EPF and EPJ. Closed symbols are for vaccinated animals. The detection limit, approximately 300 copy equivalents (Copy Eq) per ml, is indicated by the dashed line.

FIG. 3

Outcome of challenge with SIVsmE660. (A) Cell-associated virus loads measured by the number of infectious cells in PBMC. Code for PMBC load: 2, 333,333 PBMC; 3, 111,111 PBMC; 4, 37,037 PBMC; 5, 12,345 PBMC; 6, 4,115 PBMC; 7, 1,371 PBMC; 8, 457 PBMC. (B) Viral RNA loads in plasma. (C) CD4⁺ T lymphocytes. Open symbols are for control unvaccinated animals VT2 and KJ8. Closed symbols are for vaccinated animals.

FIG. 4

CTL activity to SIVmac antigens (Gag and Env) on the day of challenge with SHIV89.6p. Assays were performed following antigen-specific stimulation of PBMC (17) at the indicated effector/target ratios.

FIG. 5

CTL activity against SIVmac antigens (Gag and Env) on the day of challenge with SIVsmE660.