293 Physical Determinants of HIV Infectivity In Vitro: Brownian Motion Underlies Target Cell Limitation M. C. Strain*1, H. Levine1, D. Sheeter2, C. C. Ignacio1, D. V. Havlir3, D. D. Richman1,4, J. K. Wong1,4 1Univ of California at San Diego, La Jolla; 2Scripps Res Inst, La Jolla, CA; 3San Francisco Gen Hosp, CA; and 4Veterans Affairs San Diego Heatlhcare System, CA
Background: The infectivity of virus in vitro is typically expressed as infectious titer or multiplicity of infection. However, Brownian motion and virus lability may modulate infectivity, making these quantities highly dependent on experimental conditions.
Methods: The process of HIV infection in suspension cell culture was modeled by assuming that Brownian collisions of cells and virus are limiting. CEM cells were infected with HIVLAI, and CD8-depleted PBMCs were infected with a primary HIV-1 isolate with cell density varied by changing cell numbers or infection volumes. Infectivity was quantified by terminal dilution p24 ELISA and by HIV DNA production, assayed by real-time PCR 12 hrs after infection. IC90 values were computed based on 10-fold reduction of HIV DNA.
Results: The model predicts a linear increase in infectivity with cell density, and this result was verified in vitro. The IC90 of nevirapine was independent of cell concentration, but the IC90 of AZT increased moderately with cell density, which is consistent with lower phosphorylation rates in slowly dividing cells.
Conclusions: Our results support the use of drug resistance testing in standard cell culture. However, HIV infectivity at the high cell densities of lymphoid tissues may greatly exceed infectivity measured in vitro, which may account for incomplete viral suppression during HAART. Thus, in vitro systems allowing high target cell concentrations may better model HIV infection kinetics in vivo.
