Traditionally, experimental animals have been inoculated with viruses by the intravenous (IV), intramuscular, and the oral routes. For SIV and SHIV studies, the IV route has been extensively used in natural history and pathogenesis experiments and is the most reliable and direct way to establish infections and to disseminate virus to tissues/organs throughout the body. IV inoculated SIVs and SHIVs are rapidly delivered to secondary lymphoid tissues and the GI tract where virus replication proceeds exponentially, heralding the onset of the systemic infection. The IV route has been used to model drug user/blood transfusion, and possibly, needle-stick transmissions of HIV-1.
Mucosal inoculation of SIVs and SHIVs has become the route of choice for vaccine experiments, although it is now appreciated that virus can rapidly cross the mucosal barrier [6
]. Following the penetration of the epithelial cell layer, “founder infections” are established in CD4+
memory T cells residing in the submucosa where the progeny virus population expands [7
]. Translocation of virus and infected cells to draining lymphoid tissue/lymph nodes ushers in the systemic phase of the acute infection, resulting in an exponential increase of progeny virion production. Intrarectal inoculation of SIV and SHIV, modeling male to male transmission, typically results in a modest delay in the appearance of peak plasma viremia versus inoculation by the IV route, when comparable inoculum sizes are administered (see below). This delay presumably reflects the additional time required to penetrate the colonic columnar epithelial barrier and to establish a local infection in the submucosa. Vaginal transmission of SIV and SHIV is thought by some to be slower than when virus inoculation occurs by the IV and intrarectal routes [9
]. This additional delay could reflect the relative paucity of lymphoid tissue adjacent to submucosal founder infections and/or suppressive effects of innate immune responses in the female genitourinary tract [10
]. Representative patterns of SIV replication following low dose inoculation by different routes are shown in .
Figure 1 Representative SIV infectivity profiles in rhesus macaques following low dose inoculations by different routes. The curves shown are based on reports for inoculation of SIV by the IV (100 TCID50) , IR (930 TCID50) , and IVag (1000 TCID50) [20 (more ...)
Inter-comparisons of dose effects following inoculation by parenteral and non-parenteral routes can be problematic. First, dose size based on infectivity (viz
) is dependent on the cell types used for: 1) preparation of the virus stock; and 2) the assay used to measure infectivity. Dose size based on viral RNA copy number or Gag protein content will be affected by the amount of defective virions present in the challenge stock. Second, passage through a mucosal barrier can be affected by multiple factors compared to intravenous inoculation, where a clear relationship exists between inoculum size and the rate at which virus is disseminated systemically [12
When rhesus macaques are inoculated IV with a relatively low dose (viz
. 100 TCID50
) of SIVmac239, peak plasma viremia occurs between days 10 to 14 PI, whereas inoculation of 10,000 TCID50
accelerates this phase of the infection by 3 or 4 days ( inset) [14
]. A study of SIVmac251 titration by the IR route (106
down to 103
) reported that plasma viral RNA was initially detected 4 to 8.5 days PI, depending on the inoculum size [16
]. In the 2 of 6 animals that became infected at the dilution endpoint (103
), peak plasma viremia occurred at day 14 PI. It is worth noting that inoculation of macaques with as little as 1 TCID50
of SIV by the IV route can establish infections leading to AIDS, indicating the inefficiency of transmission by the rectal route [13
]. Another study, evaluating repeated low dose IR inoculation of SIVsmE660 or SIVmac239, reported that peak plasma viral RNA loads were attained within 1 to 2 weeks of the last unsuccessful inoculation [17
]. Single genome analyses of virus transmitted by repeated low dose rectal inoculation of uncloned SIVsmE660 or SIVmac251 revealed the presence of one or a few viruses in plasma prior to peak viremia, a result similar to that found in humans exposed to HIV-1 by mucosal routes [17
]. Taken together, these results suggest that once the mucosal barrier is penetrated by an infectious particle, the replication kinetics and systemic spread of SIV are similar to those reported for low dose inoculation by the IV route ().
Less data is available relating inoculum size and SIV infection kinetics following intravaginal (IVag) inoculation. When analyzed in the absence of hormones that cause thinning of the vaginal epithelium, inoculation with 105
led to the establishment of SIVmac251 infection in 20 of 20 animals, whereas macaques receiving 103
only became transiently viremic (intermittent virus isolations from PBMC and no detectable cell-associated viral DNA) [19
]. Based on inoculum size alone, this result would make SIV transmission by the IVag route 10-fold less efficient than virus administered rectally [17
]. In a study examining repeated low dose (1000 TCID50
) IVag inoculation of SIVmac251, rhesus monkeys became infected 2 to 3 weeks following a prior unsuccessful challenge [20
]. This is somewhat slower than the kinetics reported for low dose inoculation of SIV by the IR route. Some low dose IVag inoculated animals experienced the transient plasma viremia described above before developing a prototypical SIV systemic infection.