Mechanisms of Protection Induced by Attenuated Simian Immunodeficiency Virus II. Lymphocyte Depletion Does Not Abrogate Protection

Abstract

To determine the role that cellular immune responses play in the protection conferred by vaccination with attenuated SIVmac32H (pC8), we have attempted to deplete macaques of their CD8+ cells prior to challenge with wild-type SIVmac32H (pJ5). In two of four pC8-infected macaques, N109 and N112, a transient partial depletion of CD8+ cells by antibody treatment was achieved. On the day of challenge peripheral CD2+CD4-CD8+ cell counts were reduced by 92 and 95%, respectively, in animals N109 and N112 and their lymph nodes revealed a 46 and 58% reduction, respectively, in CD2+CD4-CD8+ cells. Two other pC8-immunized macaques, N110 and N111, treated in the same way, did not show significant depletion of CD8+ cells. None of these four pC8-immunized animals became infected when challenged with 50 MID50 of pJ5. Treatment of a further four pC8-infected and protected macaques and two naive control animals with Campath-1H antibody successfully depleted peripheral CD3+ cell counts by >99% in all treated animals. Campath-1H depletion resulted in enhanced, longer lasting lymphoid depletion. Yet subsequent challenge with 20 MID50 of pJ5 still failed to infect the pC8-immunized animals. All eight of the naive controls, including two Campath-1H-treated animals, became infected following challenge. In summary, partial depletion of circulating CD8+ cells or total lymphocytes prior to challenge failed to abrogate the protection conferred by vaccination with pC8.

CD8 + Lymphocytes Do Not Mediate Protection against Acute Superinfection 20 Days after Vaccination with a Live Attenuated Simian Immunodeficiency Virus

“…This study is part of a series that has characterized the early protection against superinfection conferred by inoculation with live nef-attenuated SIVmacC8 (3,5,48,(50)(51)(52). Previously, we showed that protection against acute superinfection with pathogenic, wild-type SIVmacJ5 can be obtained within 21 days of inoculation with live attenuated SIVmacC8, at a time when neutralizing antibodies were not present and limited CD8 ϩ T lymphocyte responses were detected (51).…”

“…Although peripheral CD8 ϩ T lymphocytes are effectively (Ͼ99%) depleted within 1 h of anti-CD8 MAb treatment, the majority of CD8 ϩ lymphocytes are found within lymphoid tissues and have been found to be resistant to effective depletion (23,28,30,52). By increasing the number of anti-CD8 MAb treatments to eight, given over a 3-week period, we were able to effectively deplete lymphoid tissue CD8 ϩ T lymphocytes in naïve macaques and extend greatly the duration of depletion.…”

“…The sensitivity of the assay is 200 SIV RNA copies per ml of plasma. Virus isolation and titration from PBMC were determined by coculture with C8166 cells; the presence of replicating virus was confirmed by syncytia identification or by antigen capture at 28 days, as previously described (52). Titers of binding antibodies to SIV envelope gp130 (EVA670; CFAR/NIBSC, Potters Bar, United Kingdom) or recombinant SIV p27 (EVA643; CFAR/NIBSC, Potters Bar, United Kingdom) were determined in heat-inactivated (56°C for 1 h) plasma samples by enzyme-linked immunosorbent assay, as previously described (4,47,53).…”

“…In addition, an inverse correlation has been reported between the precursor frequency of SIV-specific CD8 ϩ CTL responses elicited by certain vaccine approaches and virus load following challenge (20,55). Although several groups have reported a correlation between SIV-specific CD8 ϩ CTL responses in live attenuated SIV vaccinees and protection against superinfection with wild-type SIV (24)(25)(26)56), other groups have failed to corroborate such observations and dispute a role for SIVspecific CD8 ϩ CTL in mediating protection (1,32,44,50,52).…”

“…In a previous study we attempted to address the role of SIV-specific CD8 ϩ CTL responses in mediating protection against superinfection by administering a pair of rat anti-human CD8 MAbs to live attenuated SIVmacC8 vaccinees 24 h prior to challenge with wild-type SIVmacJ5 (52). All CD8 ϩ lymphocyte-depleted vaccinees resisted superinfection with wild-type SIVmacJ5, which suggests that SIV-specific CD8 ϩ CTL responses are not central to protection against superinfection observed at 35 weeks postinoculation.…”