Helper T Cells Not Necessary for Activation of Killer Cells

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LONDON (Reuters Health) Apr 26 - A brief initial exposure to antigen
may be all that is needed to stimulate naïve CD8+ T cells to undergo
several rounds of proliferation and differentiation into effector and
memory cells, according to the findings of two studies published in
the May issue of Nature Immunology.

In one study, Dr. Rafi Ahmed and Dr. Susan M. Kaech from Emory
University School of Medicine in Atlanta used a murine model of
infection and cell culture techniques to assess CD8+ T cell
development.

The researchers found that after antigenic stimulation, naïve CD8+ T
cells were committed to divide at least seven times and differentiate
into memory and effector cells. Further antigenic stimulation was not
required for the daughter cells to divide, and once the process
started it could not be interrupted.

In another study, Dr. Stephen P. Schoenberger and colleagues from La
Jolla Institute for Allergy and Immunology in San Diego, California
used an engineered antigen-presenting cell system to determine the
signalling duration requirements for priming and clonal expansion of
naïve CD8+ cytotoxic T cells.

The authors found that naïve cytotoxic T cells required as little as
2 hours of exposure to antigen-presenting cells to be committed to
division and differentiation. As in the first study, no further
antigenic stimulation was needed for daughter cells to divide and
differentiate, whether the priming was in vitro or in vivo.

In an editorial, Dr. Michael J. Bevan and Dr. Pamela J. Fink from the
University of Washington in Seattle point out that "contact with
antigen can be brief but other factors may control the overall
magnitude of the CD8+ T-cell response."

While the current findings raise important questions for future
studies, "today we are left to deal with the idea of antigen-
triggered CD8+ T cells running on autopilot," Dr. Bevan and Dr. Fink
conclude.

Nature Immunol 2001;2:381-382,415-429.


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