Quantitative Image Analysis of HIV-1 Infection in Lymphoid Tissue

A quantitative technique was developed to determine HIV-1 viral burden in two important cellular compartments in lymphoid tissues (LT). Image analysis and in situ hybridization were combined to show that in the presymptomatic stages of infection there is a large pool of about $10^8$ copies per gram of LT of viral RNA in virions on the surfaces of follicular dendritic cells, and a smaller pool of $10^3$ to $10^5$ (per gram) of infected cells with 20 to 200 copies of viral RNA. The concentration of viral RNA in these cells was found to be much less than in productively infected cells in culture, consistent with restriction of replication of HIV-1 in vivo by immune or other mechanisms. The agreement, however, between theoretical estimates of viral production in LT and clearance of virus from plasma suggest that sufficient virus could nonetheless be generated in LT to account for levels of virus detected in plasma, and that plasma assays may turn out to accurately mirror virus production in tissues. Tracking HIV-1 infection at the cellular level in tissue reservoirs provides opportunities to better understand the pathogenesis of infection and the impact of treatment.