312
Cell Growth in Microgravity
Lymphocyte locomotion through inter-
stitium is a critical stage in the immune
response to antigenic challenge. Initially,
the MACS was used to demonstrate the
suppression of lymphocyte locomotion
through collagen in modeled micrograv-
ity. Locomotion can be investigated
in vitro
using gelled type I collagen as a test matrix.
In analog culture, lymphocytes completely
lose their capacity to randomly locomote
in gelled collagen.
Experiments conducted in microgravity
onboard Shuttle missions STS-54 and STS-
56 demonstrated similar suppression of
lymphocyte locomotion and support the
relationship between orbital microgravity
and analog culture.
In a study by Cooper and Pellis, the
MACS was used to further characterize
the suppression of polyclonal activation of
T cells with PHA (phytohemagglutinin).
Results suggest that accessory cell costim-
u
la
t
ionintheformo
fcy
tok
inesisin
tac
t
in the MACS. Furthermore, the calcium-
signaling pathway may be intact during
PHA stimulation in the MACS. Therefore,
signal transduction mechanisms between
T-cell receptor engagement and down-
stream protein kinase C (PKC) activation
are sensitive to modeled microgravity. The
suppression of lymphocytic PHA response
is observed in both the lymphocytes of as-
tronauts after spaceflight and in
in vitro
experiments in spaceflight and modeled
microgravity. Although some studies sug-
gest there may be a microgravity-induced
PKC defect, T cells were fully activated to
proliferate in modeled microgravity by di-
rect activation of PKC using phorbol myris-
tate acetate (PMA) and ionomycin. These
pharmacological agents
bypass mecha-
nisms at the cell surface and impart their
effect inside the cell: PMA by mimicking 1,
2-diacylglycerol (DAG) and activating the
serine/threonine kinase PKC; ionomycin
by increasing the cytoplasmic-free calcium
ion concentrations. The ability to activate T
cells in the RWV with PMA and ionomycin
implies that all the cellular mechanisms in-
volved in T-cell activation downstream of
PKC activation and calcium ion flux are in-
tact in modeled microgravity. Some flight
studies indicate that PKC may be unaf-
fected or activated by microgravity. It is still
unc
learastohowPKCisa
f
fec
tedbym
i
-
crogravity and this requires further inves-
tigation. These results explain why PHA
responsiveness could be partially restored
by preincubating PHA-stimulated cultures
at normal gravity before exposing them to
modeled microgravity. Cells that achieved
PKC activation and calcium flux continued
on to proliferate even when they were ex-
posed to modeled microgravity. Since PKC
activation and calcium flux occur quickly
after receptor triggering, one might expect
full activation to be restored by advance
exposure to PHA cultures. Although these
events do occur within minutes, sustained
receptor triggering of up to 48 h is re-
quired for optimal lectin activation. Once
PHA primed cultures were exposed to
modeled microgravity, the signal from the
receptor was no longer sustained owing
to the microgravity-induced lesion some-
where upstream of PKC activation that
was observed with the phenomenon of
suppression of lymphocyte locomotion in
microgravity and modeled microgravity.
In addition to polyclonal and oligoclonal
models, antigen-speci±c models of T-cell
activation are also suppressed in micro-
gravity and its cell culture analog.
Currently, the results strongly suggest
that signaling pathways upstream of PKC
are affected in microgravity. An emerg-
ing hypothesis is that the microgravity-
induced lesion in T-cell activation and
lymphocyte locomotion resides between
the cell surface and PKC. There is some
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