Decay characteristics of HIV-1- infected compartments during combination therapy Perelson et al. 1997 Kelsey Collins BIOL0380 September 28, 2009
SUMMARY Analyzed decay patterns of viral load of HIV- 1-infected patients after treatment with antiretroviral agents Created mathematical model to determine relative contributions of short-lived, longlived, and latently infected cells to the viral load over time
BIOLOGY OF HIV Primarily affects vital cells in the human immune system Helper T cells (CD4+, specifically) Macrophages Dendritic cells SEM of HIV-1 budding from cultured lymphocyte
BIOLOGY OF HIV Infection leads to low levels of CD4 + T cells through 3 main mechanisms: Direct viral killing of infected cells Increased rates of apoptosis in infected cells Killing of infected CD4 + T cells by CD8 cytotoxic lymphocytes When CD4 + T cell numbers drop below a critical level, cell-mediated immunity is lost & body becomes progressively more susceptible to opportunistic infections
HIV-1 More virulent strain Easily transmitted Cause of majority of HIV infections globally Ancestry traced to SIV (simian immunodeficiency virus)
PROCEDURE Subjects: 8 HIV-1 infected patients, naïve to antiretroviral agents Subjects given antiretroviral treatment (protease inhibitor, 2 reverse transcriptase inhibitors) Measured HIV-1 RNA concentration in plasma weekly for 1 st month, then every 2 weeks using branched DNA assay
OBSERVATIONS Each patient had similar pattern of viral decay: Phase 1: initial rapid exponential decline Phase 2: slower exponential decline By 8 weeks, plasma viraemia in all patients dropped below standard detection threshold of 500 copies/ml
HYPOTHESES Rapid decline of phase 1: decay of productively infected CD4 + T cells with short t 1/2 Slower decline of phase 2: secondary source(s) are main producers of virions 2 distinct phases in data from each patient suggest there is only 1 major secondary source of virions
QUESTION What types of cells constitute 'secondary sources' and what are their relative contributions to the production of virions during the second decay phase?
KINETIC MODEL (1) dt*/dt = kvt + al δt* (2) dl/dt = fkvt μ L L (3) dm*/dt = k M VM μ M M* (4) dv/dt = NδT* + pm* - cv
RATE OF CHANGE OF T* dt*/dt = kvt + al δt* T*: # of infected CD4 + T cells k: rate constant at which T* cells are generated from uninfected CD4 + T cells V: # of virions T: # of uninfected CD4 + T cells a: rate constant at which latently infected lymphocytes are converted to productively infected cells L: # of latently infected lymphocytes δ: rate constant at which T* cells are lost
RATE OF CHANGE OF L dl/dt = fkvt μ L L L: # of latently infected lymphocytes fk: rate constant at which latently infected T cells containing infectious provirus are generated V: # of virions T: # of uninfected CD4+ T cells μ L : total rate constant of loss for latently infected lymphocytes
RATE OF CHANGE OF M* dm*/dt = k M VM μ M M* M*: # of long-lived infected cells k M : rate constant at which uninfected M cells become M* cells V: # of virions μ M : rate at which M* cells are lost
RATE OF CHANGE OF V dv/dt = NδT* + pm* - cv V: # of virions N: burst size of a T* cell T*: # of infected CD4+ T cells p: average rate of virus production per M* cell M*: # of long-lived infected cells c: rate constant of virion clearance
VIRAL DECAY MODEL The level of plasma virus after drug therapy should decay as follows: V(t) = V o [Ae -δt + Be -μ Lt + Ce -μmt +(1 A B C)e -ct ] Assumptions: HIV-1 reverse transcriptase & protease are completely inhibited by antiretroviral regimen System at steady-state before treatment, with baseline viral load V o & CD4+ cell count T o
APPLICATION OF MODEL Used nonlinear least-squares regression to fit data with 2 models derived from V(t): Long-lived infected cell model Latently-infected cell model 2 models indistinguishable: need more data
PBMC INFECTIVITY ASSAYS Used limiting dilution to measure infectivity titre of HIV-1 in PBMC at each time point Infected CD4+ T cells Long-lived productively infected cells Latently infected cells activated in vitro
PARAMETER ESTIMATES Simultaneously fit plasma viraemia & PBMC data to equations for V(t) & I(t) to yield estimates of decay rate constants
FINDINGS Long-lived infected cells are the major contributor of virions to phase 2 of plasma viraemia decay long-dashed/short-dashed line: contribution of preexisting productively infected T cells and latently infected T cells as they are gradually activated dashed line: contribution of long-lived infected cells
IMPLICATIONS 1 st direct evidence for rapid decay of productively infected cells Known compartments of virus could be completely eradicated after 2.3-3.1 years of treatment with 100%-inhibitory antiretroviral regimen Complete elimination of HIV-1 from infected person may require longer treatment period Need further study of latently infected cells that could not be experimentally activated