Immunity following vaccination can be temporarily lost due to
another infection, but replenished over time by immune cells called
memory B cells, according to a new study.
The fascinating finding comes from a study by a team at the
Medical Research Council's National Institute for Medical Research
(NIMR; now part of the Francis Crick Institute) who vaccinated mice
against flu and then infected them with malaria.
The subsequent malaria infection caused the mice to lose their
immunity to influenza. However this loss was only temporary - they
regained immunity within a few weeks.
George Kassiotis of NIMR explained: "Think of
immunological memory (in this case, antibody memory of immunity to
flu) as computer memory. You store all your photos and videos -
memories of past events - as single copies on a physical hard
drive. You can retrieve these memories as you need them, but if the
hard drive breaks down they are permanently lost.
"However, imagine that these memory files are not
static copies, but are constantly being replaced with new copies
from another source, such as an online storage vault. If the hard
drive breaks down now, you would temporarily lose your memories,
but as soon as you plug in a new hard drive, they could all be
copied back and restored."
After vaccinating the mice against flu, checking that
they had developed immunity and then infecting them with malaria,
the researchers measured their amount of flu-specific antibodies
and numbers of specific plasma cells over several weeks. The
specific plasma cells are responsible for secreting the antibodies
that provide immunity to flu.
They found that the plasma cells were replenished
over the following weeks by circulating memory B cells, a type of
white blood cell. The work shows that B memory cells are crucial -
not only for providing immunity against reinfection by a pathogen
an animal is already immune to, but also in recovery of this
immunity if it is lost due to infection by a different
pathogen.
Jean Langhorne, also of NIMR, said: "This study is
important because it directly demonstrates how an acute infection
such as malaria can cause loss of existing immunity to other
infections. This might mean that there are periods when an
otherwise protective vaccine is no longer protective.
"However, this is transient as memory B cells
replenish the pool of cells making protective antibodies. A large
pool of memory B cells is therefore very important for maintaining
and recovering protective antibody responses after
vaccination."
The results have implications for understanding the length of
time vaccination provides immunity against various diseases, which
is especially important in countries where continuous infections
are common.
The paper, Recovery of an antiviral antibody response following attrition
caused by unrelated infection, by Dorothy Ng, John Skehel,
George Kassiotis and Jean Langhorne, is published in PLOS
Pathogens.