Most people understand that when
people become infected with a pathogen, like a virus or bacterium, their immune
system provides a rapid and robust response to clear the pathogen. However, did
you know that there are many different types of immune cells? Some of our
immune cells act immediately during infection and then die, and some are long
lasting and become memory cells. Vaccines mediate protection against pathogens
by stimulating the production of immune cells, some of which eventually become
memory cells. When we get a vaccine of an attenuated or inactivated pathogen
our memory cells “remember” the pathogen, so if it attempts to invade our
bodies again the memory cells can tell the rest of the immune system how to
respond; this is called immunological memory. Because memory cells confer
long-term protection against pathogens, much effort goes in to understanding
how a cell becomes destined to be a memory cell. And while much progress has
been made in this field, the complicated signals and interactions involved in
deeming a cell a memory cell are still unknown (Kaech et al., 2002).
One important type of immune cell
is the T cell. There are many types of T cells, and one of these essential T
cells is the CD8+ T cell, sometimes called a cytotoxic or killer T cell. These
CD8+ T cells effectively “kill” regular cells in the body that have been
infected with a pathogen; this helps stop the pathogen from multiplying and
infecting more cells nearby. Because CD8+ T cells play such a central role in
preventing infection, a subset of activated CD8+ T cells during infection
become memory cells, to provide a swift response upon pathogen re-exposure. But
how do these activated CD8+ T cells know if they should be a short-term
effector cell, or to become a memory cell to stick around for the long term? A
group of researchers thinks that they have found a piece to this puzzle. A
recent study shows that the memory fate decision may be a function of the type
of activation the T cell receives.
CD8+ T cells, hereon referred to as
cytotoxic T cells or just T cells, are specific for a certain antigen. An
antigen in this context is a peptide, a piece of protein, from a pathogen that
the immune system recognizes as foreign, and serves as an activator for the
immune response. A cytotoxic T cell is activated when a foreign peptide for
which it is specific is presented to it by another immune cell called a
dendritic cell (DC) (Fig. 1). The DC encounters the pathogen, takes up the peptide, and
presents it to the naïve (inactivated) cytotoxic T cell. There are two phases
to this activation process. In the first, the DC and cytotoxic T cell make many
serial brief contacts. In the second phase a long-lasting interaction, about 30
minutes, is made between the cytotoxic T cell and DC cell. It was previously
thought that both phases were necessary to activate the cytotoxic T cell to
begin to divide, and make effector cells to go out and kill cells infected with
pathogen (Hugues et al., 2004). However, this study has found that the interactions
in phase I and II determine if a cell will become a short-term or memory
cytotoxic T cell.
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| Fig. 1. DC:T Cell Interaction |
The authors of this
paper used a mouse model to study T and DC cell interactions because mice have
a similar immune system to humans. The scientists isolated DC cells in culture,
and treated them with either high (100C) or low (1C) amounts of an antigenic
peptide. The DC cells that were 100C are considered to have a high concentration
of antigenic peptide on their surface to be presented to their cognate
cytotoxic T cell, and the 1C are considered low concentration. The scientists
then fluorescently labeled the DCs, and injected 1C or 100C DC cell populations
in to mice. Then 18 hours later, the researchers injected the mice with
fluorescently labeled cytotoxic T cells that had a receptor specific for the antigenic
peptide presented by the injected DCs. The scientists then waited a few hours
to allow the T cells to travel to the lymph node where the injected DC cells
reside. Then, to visualize the interactions between the T and DC cells, the
group used a method called multiphoton intravital microscopy. Using this
method, scientists can look through a microscope in to a tissue of a living
mouse, and watch interactions between cells. The group from this paper used
this technique to count the interactions, and length of interactions, between T
cells and DC cells in the lymph node. (**If you would like to know more about
multiphoton intravital microscopy, a link is provided at the end of this blog
post. Note it is slightly graphic.)
 |
| Fig. 2. Graphic of MP-IVM |
Using multiphoton intravital
microscopy (MP-IVM), the scientists compared the number and length of
interactions between T cells and DC cells in mice with 100C DCs or 1C DCs. They
found that in the mice injected with only 1C DCs, the T cells were able to
interact with the DC, and become activated through phase 1. Remember, phase I
consists of multiple transient interactions between the T cell and DC cell. However,
these T cells that only interacted with 1C DCs were not able to move on to
phase two and have a long-term contact with a DC. On the other hand, T cells in
mice with 100C DCs went through phase I more rapidly than the 1C DC mice, and
then were able to form a longer phase II-type interaction with a DC. The group
then asked, what are the consequences of this? Are T cells that are activated
by phase II different than T cells activated only by phase I?
To investigate this, the
researchers isolated T cells from mice that were activated by 1C DCs and 100C
DCs, and examined their numbers and effector function. They found that at 48
hours, both T cell populations proliferated equally, meaning they divided into
more cells at similar rates. However at 96 hours, the T cells that were
activated by 100C DCs were much more numerous than T cells activated by 1C DCs.
Next they wanted to see if their effector functions were different. They looked
at secretion of interferon (IFN) and other cytokines that are secreted by T
cells during infection as part of the immune response. At 48 hours cytokine
production between the 100C-activated and 1C-activated T cell populations was
similar. However, at 96 hours, T cells activated by 100C DCs were secreting
more cytokines and IFN. This led the group to posit that perhaps T cells
activated by DCs with a higher concentration of peptides, 100C DCs in this
study, were activated for longer and could perhaps be part of the memory cell
population after infection.
To determine if the types of
activated T cells were different in terms of gene expression, the group used a
technique called a microarray to probe for activated genes. They did a
microarray on 100C DC- and 1C DC-activated T cells at 24 hours to determine
differences in gene expression. They found that the expression of about 500
genes were different as early as 24 hours for the two groups of activated T
cells! The T cells that were activated by 100C DCs, thus going through both
phase I and II activation, expressed a number of genes associated with memory
cells, whereas the 1C-DC activated T cells did not. This finding led them to
believe that T cells that undergo only phase I activation cannot become memory
cells, but T cells that are activated by phase I and II can. To verify this finding,
they did another experiment in mice.
The scientists injected T cells
activated by 1C DCs into some mice, and 100C DCs in another group of mice.
After 30 days, they inoculated the mice with the antigenic peptide that the T
cells were specific for. The mice that had 1C DC-activated T cells showed a
slow and weak response to the pathogen. However, the mice with T cells
activated by the 100C DCs exhibited a swift and robust response to the
pathogen. This experiment verified that the T cells activated by both phases
were able to form memory cells, but T cells activated only by transient phase I
interactions were not. So what does this all mean?
This study’s findings can be broken
down in to two main parts. First, as mentioned earlier, it was previously thought
that T cells could only be activated if they went through phase I and II
activation. However, this study showed that T cells can be activated by just
phase I interactions with DCs. They showed that low concentration peptide on
DCs cause phase I interaction, and high concentration peptide on DCs cause
phase I and II interaction. These two populations of activated T cells have
similar abilities to fight pathogen at 48 hours, but the T cells activated by
100C DCs are more effective from 96 hours on. Their second major finding was
that this activation pattern, either phase I or phase I and II, is a
determinant for memory fate decisions. So what does this mean for the future
and why is it important?
Since memory cells confer the
protection of vaccines, it is important to understand how cells become memory
cells. This finding can help scientists to optimize vaccines, by helping them
determine the concentration of antigenic peptides that need to be injected for
optimal memory cell production. In addition the findings in this paper lead us
to ask some more questions that could be addressed in subsequent studies to
better understand memory fate decisions. At 24 hours, they found that the two
types of activated T cells already had “decided” if they were going to be
memory or short-lived T cells. In the future, we could look at time points
earlier than 24 hours to see how soon gene expression changes to become
short-lived or memory cells in the two T cell populations. Additionally, not
all of the T cells activated by high peptide concentration DCs become memory
cells. So the next question is how do the T cells that are capable of becoming
memory cells, i.e. cells activated by phase I and II decide to become memory
cells? The findings of this paper and future studies addressing questions like
the ones posed above will help us to gain a better understanding of memory cell
fate decisions, and will have implications for developing more effective
vaccines to protect us from disease.
Primary Source:
Henrickson, S.E.,
Perro, M., Loughhead, S.M., Senman, B., Stutte, S., Quigley, M., Alexe, G.,
Iannacone, M., et al. (2013). Antigen Availability Determines CD8+ T
Cell-Dendritic Cell Interaction Kinetics and Memory Fate Decisions. Immunity, 39, 496-507.
Secondary Sources:
Hugues, S.,
Fetler, L., Bonifaz, L., Helft, J., Amblard, F., & Amigorena, S. (2004).
Distinct T cell dynamics in lymph nodes during the induction of tolerance and
immunity. Nat. Immunol., 5, 1235-1242.
Kaech, S.M.,
Hemby, S., Kersh, E., & Ahmed, R. (2002). Molecular and functional
profiling of memory CD8 T cell differentiation. Cell, 111, 837-851.
Picture Sources:
1. Mak, T.W., Saunders,
M.E. (2011). Primer to The Immune Response. Amsterdam: Elsevier.
2. Miller, M.J.,
Wei, S.H., Cahalan, M.D., & Parker, I. (2002). Autonomous T cell
trafficking examined in vivo with
intravital two-photon microscopy. PNAS,
100, 2604-2609.
**Multiphoton Intraveital Microscopy (MP-IVM) Video**
http://www.jove.com/video/3504/intravital-imaging-of-the-mouse-thymus-using-2-photon-microscopy
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