Conserved patterns of coreceptor use in other hosts of S

In this thesis, I examined SIV infection in a single natural host primate species and a single non-natural host primate species. Defining alternative coreceptor-mediated SIV infection as a feature distinguishing additional natural host species from CCR5-dependent SIV infection in additional non-natural host species will provide further evidence of CCR5-independent coreceptor

use as a possible determinant of CD4+ T cell homeostasis in the presence of high-level viral

replication. We hypothesize that alternative coreceptor-mediated SIV infection is a conserved feature in natural hosts whereas CCR5-restricted SIV infection is a conserved feature in non- natural hosts and human hosts that progress to AIDS.

There is evidence of CCR5-independent and alternative coreceptor-dependent SIV infection in

other natural hosts. African primate red-capped mangabeys (Cercocebus torquatus) are

presumed not to progress to AIDS following endemic SIVrcm infection (5, 33). SIVrcm infected

CD4+ cell lines engineered to express human CCR2 or human CXCR6 but not human CCR5 in

vitro (5, 16). After SIVrcm was passaged in macaque cells, SIVrcm gained the ability to infect

CD4+ cell lines expressing human CCR4 but still lacked the ability to infect CD4+ cell lines

expressing CCR5 (32). Similar experiments were not performed with coreceptors of red-capped

mangabey origin and further studies of SIVrcm infection and replication in primary cells ex vivo

are needed. However, these data suggest that SIVrcm infection in red-capped mangabeys may be entirely CCR5-independent and alternative coreceptor-dependent.

SIVagm infections of African green monkeys (Chlorocebus) also exhibit evidence of CCR5-

independence and alternative coreceptor use. African green monkeys are clearly natural hosts of

SIVagm and infection results in high viral loads without chronic immune activation, CD4+ T cell

loss, or progression to AIDS (44, 55, 63, 69, 70, 73). Prior experiments found that SIVagm

infected CD4+ transfected cell lines via alternative coreceptors CCR5, CXCR6, and GPR15 of

human origin(35, 55, 56) and that Maraviroc blocked only a fraction of SIVagm infection in TZM-

bl cells (27, 35). We recently observed that African green monkey sub-species sabaeus CXCR6 (agmCXCR6) was a robust alternative coreceptor of a sabaeus SIVagm.sab Env in transfected

293T cells (Figure 5.1, unpublished, data courtesy of K. Sheehan-Wetzel). Alternative

coreceptors sabGPR15 and sabGPR1 also supported moderate to low SIVagm Env-mediated

infection in vitro. Thus, the overall pattern of alternative coreceptor use by SIVagm appears

similar to that of SIVsmm/SIVmac in vitro. Additional in vitro experiments with a diverse panel of

primary sabaeus SIVagm Env are needed; unfortunately, many available SIVagm isolates were passaged extensively in human cell lines and may no longer resemble coreceptor use by wild SIVagm viruses (1, 35, 47). Further studies of SIVagm infection and replication in primary cells treated with coreceptor antagonists are also needed.

Human HIV-1 infection is clearly pathogenic. In the majority of untreated cases, HIV-1 leads to

CD4+ T cell loss and progress to AIDS marked by increased susceptibility to opportunistic

infections. The HIV-1 virus itself is CCR5-tropic and, like SIVsmm/SIVmac infection of rhesus macaques, human HIV-1 infection is highly CCR5-dependent both at the point of infection and throughout early stages of infection. In a subset of cases, HIV-1 can acquire the use of CXCR4. However, CXCR4 use is clearly not required for pathogenesis and progression to AIDS.

SIVcpz-infected chimpanzees exhibit signs of pathogenesis including loss of CD4+ T cells in the

spleen and lymph node, as well as an increased death hazard relative to uninfected chimpanzees

(48). Pathogenic SIVcpz infection of chimpanzees (Pan troglodytes) may also be CCR5-

dependent. Preliminary data suggest SIVcpz Env display robust infection through chimpanzee

CCR5 and little to no infection through CXCR6, GPR15, or other putative coreceptors in vitro (K.

Sheehan-Wetzel, unpublished, data not shown). Further experiments will be needed to fully

define SIVcpz coreceptor utilization in vitro and ex vivo. SIVcpz is of special interest because the

two SIV from which it arose are presumed to be non-pathogenic in their respective African hosts

(3,  19,  33)

pandemic (80) (see Chapter 1, Introduction, Figure 1.1). It not known if African greater spot-

nosed monkey (Cercopithecus nictitans) SIVgsn Env, the precursor to SIVcpz Env, use CCR5 or

alternative coreceptors for infection and replication in vitro or ex vivo. We hypothesize that

SIVgsn infection is at least partially CCR5-independent. Further evidence that greater spot- nosed monkeys do not progress to AIDS as well as identification of SIVgsn alternative coreceptors may define a bottleneck leading to CCR5-tropism, CCR5-dependence, and pathogenesis during transmission of SIVgsn to chimpanzees.

It is intriguing that multiple convergent mechanisms may lead to CCR5-dependence in non- natural hosts and CCR5-independence in natural hosts of SIV and HIV-1. Notably, the

mechanisms leading to highly CCR5-dependent human HIV-1 infection and highly CCR5-

dependent rhesus macaque SIV infection are very different. Whereas host factors dictate CCR5- dependence of multi-tropic SIV in rhesus macaques, the CCR5-tropic HIV-1 virus itself dictates CCR5-dependence in humans. Similarly, divergent mechanisms may lead to CCR5-

independence in hosts that do not progress to AIDS. SIVrcm CCR5-independence is likely mediated by the SIVrcm virus, which appears not use CCR5 at all, whereas SIVsmm and SIVagm CCR5-independence are a function of the multi-tropic SIV (26, 35, 79), the exceedingly low

expression of CCR5 on CD4+ T cells (72, 74), and the expression of functional CXCR6 in sooty

mangabeys and African green monkeys.

Lack of CXCR6-mediated SIV infection may be a common feature in non-natural