Adult macaques do not develop disease after infection with a nef deletion mutant of the simian immunodeficiency virus (SIV) and are protected against challenge with pathogenic virus. This finding led to the proposal to use nef-deleted viruses as live, attenuated vaccines to prevent human acquired immunodeficiency syndrome (AIDS). In contrast, neonatal macaques developed persistently high levels of viremia after oral exposure to and SIV nef, vpr, and negative regulatory element (NRE) deletion mutant. Severe hemolytic anemia, thrombocytopenia, and CD4+ T cell depletion were observed, indicating that neither nef nor vpr determine pathogenicity in neonates. Because such constructs have retained their pathogenic potential, they should not be used as candidate live, attenuated virus vaccines against human AIDS.

https://science.sciencemag.org/content/sci/267/5205/1820.full.pdf

Pathogenicity of live, attenuated SIV after mucosal infection of neonatal macaques

Infection of macrophage lineage cells is a feature of primate lentivirus replication, and several properties of primate lentiviruses seem to have evolved to promote the infection of macrophages. Here we demonstrate that the accessory gene product Nef induces the production of two CC-chemokines, macrophage inflammatory proteins 1alpha and 1beta, by HIV-1-infected macrophages. Adenovirus-mediated expression of Nef in primary macrophages was sufficient for chemokine induction. Supernatants from Nef-expressing macrophages induced both the chemotaxis and activation of resting T lymphocytes, permitting productive HIV-1 infection. These results indicate a role for Nef in lymphocyte recruitment and activation at sites of virus replication.

HIV-1 Nef mediates lymphocyte chemotaxis and activation by infected macrophages.

HIV-1 Nef and the unrelated mouse leukaemia virus glycosylated Gag (glycoGag) strongly enhance the infectivity of HIV-1 virions produced in certain cell types in a clathrin-dependent manner. Here we show that Nef and glycoGag prevent the incorporation of the multipass transmembrane proteins serine incorporator 3 (SERINC3) and SERINC5 into HIV-1 virions to an extent that correlates with infectivity enhancement. Silencing of both SERINC3 and SERINC5 precisely phenocopied the effects of Nef and glycoGag on HIV-1 infectivity. The infectivity of nef-deficient virions increased more than 100-fold when produced in double-knockout human CD4(+) T cells that lack both SERINC3 and SERINC5, and re-expression experiments confirmed that the absence of SERINC3 and SERINC5 accounted for the infectivity enhancement. Furthermore, SERINC3 and SERINC5 together restricted HIV-1 replication, and this restriction was evaded by Nef. SERINC3 and SERINC5 are highly expressed in primary human HIV-1 target cells, and inhibiting their downregulation by Nef is a potential strategy to combat HIV/AIDS.

SERINC3 and SERINC5 restrict HIV-1 infectivity and are counteracted by Nef

A substantial risk in using live attenuated, multiply deleted viruses as vaccines against AIDS is their potential to induce AIDS. A mutant of the simian immunodeficiency virus (SIV) with large deletions in nef and vpr and in the negative regulatory element induced AIDS in six of eight infant macaques vaccinated orally or intravenously. Early signs of immune dysfunction were seen in the remaining two offspring. Prolonged follow-up of sixteen vaccinated adult macaques also showed resurgence of chronic viremia in four animals: two of these developed early signs of disease and one died of AIDS. We conclude that this multiply deleted SIV is pathogenic and that human AIDS vaccines built on similar prototypes may cause AIDS.

Live attenuated, multiply deleted simian immunodeficiency virus causes AIDS in infant and adult macaques

Cell-surface expression of CD4 reduces HIV-1 infectivity by blocking Env incorporation in a Nef- and Vpu-inhibitable manner

Serine phosphorylation-independent downregulation of cell-surface CD4 by nef.

By considering the dynamic relationship between retroviruses and their hosts, we have developed a unifying hypothesis to explain such disparate clinical phenomena as differential pathogenicity of a given virus in adults and neonates, transient infection with clearance of provirus-containing cells, long-term non-progression and vaccine effects of fully pathogenic viruses. The threshold hypothesis predicts that an opportunity exists during acute retroviral infection to influence the ultimate clinical outcome: if virus replication is kept below threshold by any means, including drug therapy or passive immunoprophylaxis with neutralizing antibodies, the host will prevail and win the race.

Murine and simian retrovirus models: the threshold hypothesis.

The pandemic of the acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus type 1 (HIV-1), requires rapid development of effective therapy and prevention Analysis of candidate anti-HIV-1 drugs in animals is problematic since no ideal animal model for HIV-1 infection and disease exists For many reasons, including small size, availability of inbred strains, immunological reagents, and lymphokines, murine systems have been used for in vivo analysis of antiretroviral agents Here we review currently available murine models involving HIV-1 in transgenic mice and in chimeric mice reconstituted with human cells, as well as murine systems using retroviruses of the subfamily Oncovirinae rather than Lentivirinae We report our results on various antiretroviral treatment strategies, including chemoprophylaxis after acute retroviral exposure, therapy of chronic viremia, quantitative analysis of combination therapy, and therapy during pregnancy and in the neonatal period aimed at preventing viremia in the offspring Due to our highly effective postexposure treatment protocols with 3′-azido-3′-deoxythymidine (zidovudine) combined with recombinant human interferon-αA/D, retrovirus-inoculated mice developed immunity to the virus to which they were exposed, which will allow us to determine the nature of protective antiretroviral immunity in inbred mice

https://cancerres.aacrjournals.org/content/canres/50/17_Supplement/5618s.full.pdf

Murine Models for Evaluating Antiretroviral Therapy

Murine models with type C murine leukemia viruses have been used to develop major new prophylactic and therapeutic strategies in vaccination, drug therapy of acute virus exposure and chronic viremia, combination therapy, prevention of maternal transmission, and therapy targeted to the central nervous system. Transgenic mice expressing either the whole human immunodeficiency virus type 1 (HIV-1) provirus or subgenomic sequences allow the in vivo analysis of selected HIV-1 functions. The full replicative cycle of HIV-1 can be studied in human/mouse chimerae which were created by transplanting human hematolymphoid cells into SCID mice. The chimeric SCID mouse models have been used successfully to evaluate anti-HIV-1 drugs. The role of the various murine retrovirus systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.

Prem L. Sharma

Papers

Serine phosphorylation-independent downregulation of cell-surface CD4 by nef.

Murine and simian retrovirus models: the threshold hypothesis.

Murine Models for Evaluating Antiretroviral Therapy

Development of Antiviral Treatment Strategies in Murine Models

Live attenuated HIV as a vaccine for AIDS: pros and cons