Download The role of mRNA processing bodies (P

The role of mRNA processing bodies (P-bodies) in CD8+ memory T cells
Zinaida Tebaykina, Kate Choi, Lisa C. Osborne, Ninan Abraham, and Michael R. Gold
Department of Microbiology and Immunology, University of British Columbia, Vancouver,
British Columbia, Canada
mRNA processing bodies (P-bodies) are granular cytoplasmic aggregates that contain
translationally-repressed mRNAs, as well as repressor proteins (GW182, RCK/p54, and DCP1a),
and facilitate miRNA-mediated mRNA storage or degradation. The partitioning of mRNAs
between a translationally-competent cytoplasmic pool and a translationally-repressed P-body
pool could be an important mechanism for dynamically controlling the synthesis of key proteins.
Although P-bodies have not been characterized in lymphocytes, we hypothesized that storage of
spliced mRNAs in P-bodies could contribute to the ability of memory T cells to mount rapid
recall responses and secrete cytokines within minutes of TCR engagement. Using flow
cytometry, immunoblotting, and confocal microscopy, we found that murine and human T and
B-cell lines, as well as primary murine T and B lymphocytes express the P-body markers
GW182, RCK/p54, and DCP1a, and that these proteins concentrated in discrete cytoplasmic
granules. The levels of these proteins increased in response to antigen receptor-induced
activation of the cells, and co-localization analysis showed that there are multiple subsets of Pbodies containing different combinations of the GW182, RCK/p54, and DCP1a proteins. This
raises the possibility that P-bodies with different protein compositions have distinct functional
properties. To explore the role of P-bodies in the recall response, we utilized the OT-I model to
generate effector and memory CD8+ T cells in vitro. Compared to naïve CD8+ T cells from OT-I
mice, effector T cells had elevated levels of P-body markers and a greater number of P-bodies.
In contrast, memory T cells had similar numbers of P-bodies as naïve cells, but contained
substantially larger RCK/p54 granules. Based on previous reports that memory CD8+ T cells
contain high levels of spliced mRNA encoding the chemokine RANTES, and can secrete
RANTES within 30 minutes of TCR engagement, we hypothesized that memory T cells store
RANTES mRNA in P-bodies so that they can mount rapid recall responses. Although memory T
cells contained greater amount of RANTES mRNA than naïve cells, RANTES mRNA did not
co-localize with P-bodies in memory cells, and instead was more frequently associated with Pbodies in naïve cells. The exclusion of RANTES mRNAs from P-bodies in memory T cells may
prevent the degradation of these pre-synthesized mRNAs and allow memory cells to rapidly
secrete RANTES and initiate immune responses to secondary infection.
Funding provided by the Canadian Institutes of Health Research.