One of the obstacles to eliminating HIV from the body is the
persistence of the virus in a latent form in long-lived memory CD4 T cells.
Memory T cells are generated by the activation, division and differentiation of
naïve T cells in response to an encounter with an antigen (typically an
infectious agent). While most activated T cells produced during the initial
battle with an infectious agent die in a matter of days (a process called
activation-induced cell death or AICD), a subset return to a “resting” state
and persist as memory T cells ready to rapidly respond if the same antigen is
re-encountered. Evidence indicates that integration of HIV DNA into the genome
of CD4 T cells as they de-activate and become resting memory cells accounts for
the development of viral latency in this cell population (a process described
in an excellent minireview by
latency expert Bob Siliciano and colleagues). Lending plausibility to this scenario, a study
by John Zaunders group, covered on the blog previously, has shown that HIV appears to preferentially
infect developing memory CD4 T cells (which can be identified by expression of
the IL-7 receptor, CD127).
Despite the evidence that HIV latency in CD4 T cells is intimately
connected to the normal behavior of the cells during an immune response, attempts to
model the phenomenon in vitro have typically relied on laboratory cell lines which
may not accurately reflect what occurs in vivo. To try and address this
challenge, a team of researchers at the Institute for Human Virology in
Baltimore has now developed a system that attempts to recapitulate the
development of CD4 T cell memory in vitro. The system involves activating naïve
cells, maintaining them in culture for 10-15 days and then nurturing the resultant
memory cells with the cytokine IL-7 for a period of 21-28 days until the cells
display the signature markers of quiescent, resting memory CD4 T cells.
Using this approach, the researchers were able to
successfully generate latently infected memory CD4 T cells. Upon restimulation,
these cells resumed HIV production and also exhibited a rate of death that
markedly exceeded the degree of activation-induced cell death observed in
parallel cultures of uninfected memory CD4 T cells. The researchers note that
this finding suggests that their system may have utility not only for
investigating viral latency, but also for studying mechanisms of HIV-driven T
cell death.
The Journal of Immunology, 2008, 181: 7713-7720.
An In Vitro System to Model the Establishment and
Reactivation of HIV-1 Latency
Alessandra Marini, Jill M. Harper and Fabio Romerio1
Institute of Human Virology, University of Maryland School
of Medicine, Baltimore, MD 21201
HIV-1 establishes latency primarily by infecting activated
CD4+ T cells that later return to quiescence as memory cells. Latency allows
HIV-1 to evade immune responses and to persist during antiretroviral therapy,
which represents an important problem in clinical practice. The lack of a valid
cellular model to study HIV-1 latency has hindered advances in the
understanding of its biology. In this study, we attempted to model HIV-1
latency using human primary CD4+ T cells infected in vitro with HIV-1 after
activation with Ag-loaded dendritic cells and then brought back to quiescence
through a resting phase in the presence of IL-7. During the resting phase,
expression of cellular activation markers disappeared and cell proliferation
and viral replication ceased, but resumed following restimulation of rested
cells with Ag or mAbs directed to CD3/CD28. In addition, higher cell death
rates were observed in HIV-1-infected than uninfected cultures during secondary
but not primary stimulation. Thus, this system may allow us to study the
biology of HIV-1 latency, as well as the mechanisms of CD4+ T cell death
following HIV-1 reactivation.
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