Characterization of a novel simian immunodeficiency virus (SIV) from L'Hoest monkeys (Cercopithecus l'hoesti): implications for the origins of SIVmnd and other primate lentiviruses

Abstract

The human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) appear to have originated by cross-species transmission of simian immunodeficiency virus (SIV) from asymptomatically infected African primates. Few of the SIVs characterized to date efficiently infect human primary lymphocytes. Interesting, two of the three identified to infect such cultures (SIVsm and SIVcpz) have appeared in human populations as genetically related HIVs. In the present study, we characterized a novel SIV isolate from an East African monkey of the Cercopithecus genus, the l'hoest monkey (C. l'hoesti), which we designated SIVlhoest. This SIV isolate efficiently infected both human and macaque lymphocytes and resulted in a persistent infection of macaques, characterized by high primary virus load and a progressive decline in circulating CD4 lymphocytes, consistent with progression to AIDS. Phylogenetic analyses showed that SIVlhoest is genetically distinct from other previously characterized primate lentiviruses but clusters in the same major lineage as SIV from mandrills (SIVmnd), a West African primate species. Given the geographic distance between the ranges of l'hoest monkeys and mandrills, this may indicate that SIVmnd arose through cross-species transmission from close relatives of l'hoest monkeys that are sympatric with mandrills. These observations lend support to the hypothesis that the primate lentiviruses originated and coevolved within monkeys of the Cercopithecus genus. Regarded in this light, lentivirus infections of primates not belonging to the Cercopithecus genus may have resulted from cross-species transmission in the not-too-distant past.

FIG. 1

In vitro growth characteristics of SIVlhoest and other primate lentiviruses in human and macaque PBMC. The replication of SIVlhoest, SIVagm, SIVsm, SIVsyk, and HIV-1 as assessed by RT activity in culture supernatant is shown graphically for macaque (A) and human (B) PBMC. For this assay, equivalent amounts (based on RT activity of stocks) of the following viruses were used: SIVlhoest-P, SIVagm155-4, SIVsmE543-3, SIVsyk/cm173, and HIV-1_IIIB. Since RT activity was never observed in SIVsyk-infected cultures, the symbols are superimposed by values for the negative control.

FIG. 2

Characteristics of SIVlhoest infection of pigtailed macaques. (A) Kinetics of sequential alterations in CD4 lymphocytes in the peripheral blood during the first 16 weeks postinoculation in eight inoculated macaques, where the open symbols and dotted lines indicate animals inoculated with the PBMC isolate and solid lines and black symbols indicate animals inoculated with the splenic isolate. (B) Kinetics of mean CD4 lymphocyte numbers with standard deviations are compared between SIVlhoest-inoculated macaques and a cohort of six macaques inoculated with SIVsmE660, demonstrating that SIVlhoest induces a profile in declining CD4 lymphocyte numbers similar to that observed with an AIDS-inducing SIV isolate.

FIG. 3

Kinetics of virus expression in lymph nodes by ISH. (A1 and A2) ISH of lymph node biopsy sample obtained 1 week postinoculation, demonstrating high virus expression. Magnification, ×6 (top) and ×55 (bottom). (B1 and B2) Lymph node biopsy sample obtained 2 weeks after inoculation, showing a reduction in the numbers of SIV-positive cells relative to that observed at 1 week. Magnification, ×6 (top) and ×55 (bottom). (C1 and C2) ISH of a lymph node biopsy sample obtained 4 weeks after inoculation, showing a further reduction in the numbers of SIV-positive cells, with diffuse hybridization localized in the crescentric distribution of follicular dendritic cells within the germinal center consistent with trapping of immune complexes containing SIV on dendritic cells. Magnification, ×6 (top) and ×55 (bottom).

FIG. 4

Comparison of the predicted protein sequence of the surface subunit (SU) of the envelope of SIVlhoest and SIVmnd reveals remarkable conservation of cysteine residues and regions such as the V3 loop analog and CD4 binding domain. Conserved cysteines are indicated by ∗, and variable cysteine residues are indicated by a ∗ above the top sequence. Potential N-linked glycosylation sites are underlined. The predicted sequence of gp120 of the SIVlhoest-7 molecularly cloned virus derived from a PBMC isolate is shown on the top (lhoest-7). Substitutions relative to this sequence in the predicted sequence of a clone of envelope amplified directly from the spleen of this monkey (lhoest-S) and the SIVmnd/GB-1 clone are shown aligned below. Dots indicate amino acid identity at a residue, and a dash indicates a gap introduced to optimize alignment. Variable regions analagous to those observed in HIV-1 and other SIVs are indicated, and the cleavage site for the transmembrane glycoprotein (TM) is shown.

FIG. 5

Diversity plot comparing SIVlhoest with representatives of each of the five major lineages of primate lentiviruses, i.e., SIVmnd, SIVsyk, SIVsm, SIVcpz, and SIVagm (SIVver). Protein sequence difference is plotted for windows of 200 amino acids moved in steps of 10.

FIG. 6

Phylogenetic relationship of SIVlhoest to other primate lentiviruses. The tree was derived by maximum likelihood analysis of a concatenated Gag-Pol-Env protein alignment (see text for details). A tree derived by neighbor-joining analysis differed in no significant way. Stars indicate that the clade to the right was found in 100% of bootstrap replicates of the neighbor-joining analysis. Horizontal branch lengths are drawn to scale, with the bar indicating 0.1 amino acid replacement per site.

FIG. 7

A schematic view of Africa, showing the ranges occupied by l’hoest monkeys (C. l’hoesti l’hoesti) and their close relatives, preussis monkeys (C. preussi preussi) and suntailed monkeys (C. solatus); the distribution of mandrills (Mandrillus sphinx) is indicated by cross-hatching. A representative l’hoest monkey is shown.