The U3 promoter and the nef gene of simian immunodeficiency virus (SIV) smmPBj1.9 do not confer acute pathogenicity upon SIVagm

Mutation of a diacidic motif in SIV-PBj Nef impairs T-cell activation and enteropathic disease

Background

The non-pathogenic course of SIV infection in its natural host is characterized by robust viral replication in the absence of chronic immune activation and T cell proliferation. In contrast, acutely lethal enteropathic SIVsmm strain PBj induces a strong immune activation and causes a severe acute and lethal disease in pig-tailed macaques after cross-species transmission. One important pathogenicity factor of the PBj virus is the PBj-Nef protein, which contains a conserved diacidic motif and, unusually, an immunoreceptor tyrosine-based activation motif (ITAM).

Results

Mutation of the diacidic motif in the Nef protein of the SIVsmmPBj abolishes the acute phenotype of this virus. In vitro, wild-type and mutant PBj (PBj-Nef202/203GG) viruses replicated to similar levels in macaque PBMCs, but PBj-Nef202/203GG no longer triggers ERK mitogen-activated protein (MAP) kinase pathway including an alteration of a Nef-associated Raf-1/ERK-2 multiprotein signaling complex. Moreover, stimulation of IL-2 and down-modulation of CD4 and CD28 were impaired in the mutant virus. Pig-tailed macaques infected with PBj-Nef202/203GG did not show enteropathic complications and lethality as observed with wild-type PBj virus, despite efficient replication of both viruses in vivo. Furthermore, PBj-Nef202/203GG infected animals revealed reduced T-cell activation in periphery lymphoid organs and no detectable induction of IL-2 and IL-6.

Conclusions

In sum, we report here that mutation of the diacidic motif in the PBj-Nef protein abolishes disease progression in pig-tailed macaques despite efficient replication. These data suggest that alterations in the ability of a lentivirus to promote T cell activation and proliferation can have a dramatic impact on its pathogenic potential.

Stable Transduction of Primary Human Monocytes by Simian Lentiviral Vector PBj

Despite the ability to infect nonproliferating cells, current lentiviral vectors are inefficient at mediating gene transfer into quiescent primary human cells such as monocytes. Here, a replication-incompetent vector based on a molecular clone of simian immunodeficiency virus strain PBj (SIVsmmPBj1.9) was generated that, in contrast to lenti- and gamma-retroviral control vectors, enabled transfer of heterologous genes into human diploid fibroblasts and cell lines blocked in the G(0) phase of the cell cycle. Moreover, freshly isolated human monocytes refractory to HIV-1-derived vectors were efficiently transduced by the PBj vector independent of the viral Nef protein. Stable chromosomal integration of PBj-derived viral expression vectors was verified in transduced cells. The capability of the PBj vector to transduce quiescent cells such as unstimulated primary human monocytes is an important extension of human gene therapy perspectives.

Impact of viral factors on very early in vivo replication profiles in simian immunodeficiency virus SIVagm-infected African green monkeys

To better understand which factors govern the levels of viral loads in early lentiviral infections of primates, we developed a model that allows distinguishing between the influences of host and viral factors on viremia. Herein we report that two species of African green monkeys (Chlorocebus sabaeus and C. pygerythrus) infected with their respective wild-type simian immunodeficiency virus SIVagm viruses (SIVagm.sab92018 and SIVagm.ver644) consistently showed reproducible differences in viremia during primary infection but not at later stages of infection. Cross-infections of SIVagm.sab92018 and SIVagm.ver644 into, respectively, C. pygerythrus and C. sabaeus revealed that the dynamics of viral replication during primary infection were dependent on the viral strain used for the infection but not on the host. Hence, the kinetics of SIVagm.sab92018 and SIVagm.ver644 were similar in both sabaeus and vervet animals, indicating that the difference in viremia levels between the two groups during the early phase of infection was not associated with the host. Coreceptor usage for these two strains showed a larger coreceptor repertoire for SIVagm.sab92018, which is able to efficiently use CXCR4 in addition to CCR5, than for SIVagm.ver644, which showed a classical CCR5 coreceptor usage pattern. These differences could not be explained by different charges of the V3 loop for SIVagm.sab92018 and for SIVagm.ver644. In conclusion, our study showed that the extent of virus replication during the primary infection is primarily dependent on viral determinants.

Pathogenicity and comparative evolution in vivo of the transitional quasispecies SIVsmmPBj8

During 14 months of infection of a pig-tailed macaque, the acutely lethal simian immunodeficiency virus SIVsmmPBj14 (SIV-PBj14) evolved from the minimally pathogenic strain SIVsmm9. The virus isolated at 8 months (SIV-PBj8) exhibited properties of both SIVsmm9 and SIV-PBj14, indicating that a phenotypic transition occurred between 6 and 10 months. To assess the influence that this new composition of biologic properties might have on pathogenicity, three pig-tailed macaques were inoculated intravenously with SIV-PBj8. Although no animals developed the severe acute disease syndrome typical of SIV-PBj14, all had high levels of viremia and died of AIDS at 4, 10. 5, and 32 months. Characterization of the SIV-PBj8-derived quasispecies that evolved in these macaques showed that at 4 days after inoculation, viruses from all three animals exhibited in vitro properties different from those of the inoculum. By 4 months, the initial phenotypic profiles had changed, with the quasispecies in plasma from the animal (J90232) that died at this time most closely resembling SIV-PBj14, not SIV-PBj8. Phylogenetic trees of the gp41/Nef region of viruses in 4-month plasma from J90232 revealed three distinct populations with high bootstrap values: one group branched with SIVsmm9, one with SIV-PBj14, and one with SIV-PBj8 (ratio of clones, 5:9:5). Nucleotide sequence analysis suggested that some members of the original SIV-PBj8 quasispecies may have been evolving toward a SIV-PBj14-like genotype at the time macaque J90232 died. The use of SIV-PBj8, which was more pathogenic than SIVsmm9, but less pathogenic than SIV-PBj14, may provide the optimal genetic background on which to identify the minimal, multigenic determinants of the SIV-PBj14 phenotype. The results of our studies on SIV-PBj14 indicate that in some, but not all, cases of primate lentivirus infection more pathogenic variants evolve, selectively proliferate, and more than likely contribute to disease progression.