AT the recent Elanco Immunity
Science Symposium held in Vienna,
Professor James Roth from Iowa
State University described the latest
advances in the understanding of
the immune system with reference
to dairy cows.
Professor Roth was joined by five
other professors from around Europe
to review and collate information on
this exciting area of research for an
invited
audience of
vets and
scientists
from around
Europe.
Their
presentations
shed new
light on what has become a dynamic
and rapidly advancing area of
veterinary medicine.
Around the time of parturition –
referred to as the vital 90 days
covering the 60 days pre- and 30 days
post-calving – the immune system
faces significant challenges and it is
common for these animals to be
affected by production-related diseases
at this time.
There is potential to improve the
veterinary response to these animals by
being aware of the degree to which
these animals are compromised and
relating this to our understanding of
disease processes around the
periparturient period.
Innate understanding
The immune system is made up of
innate (native) and adaptive (acquired)
responses. The innate system produces
an immediate response after being
activated by danger signals. Innate
responses are not antigen specific and
are required to activate adaptive
mechanisms.
The adaptive response is acquired,
taking place 10-14 days after exposure.
Adaptive responses are antigen specific
and through “memory” activate the
innate system. While adaptive
responses are based around antibody
and cell-mediated responses, the innate
system consists of natural defensive
barriers, phagocytes and neutrophils,
natural killer cells, cytokines,
complement and antimicrobial
peptides.
Veterinary clinical thinking often
focuses on the adaptive response but
interest in the capacity of the innate
system is increasing, as more is
understood about its unique qualities.
In the face of microbial invasion, sentinel cells, such as macrophages and
dendritic cells, secrete cytokines –
interleukins (IL1, IL6), tumour
necrosis factor alpha (TNFalpha) and
HMGB1 (high-mobility group protein
B1, an intracellular cytokine). In the
case of a weak response, the action is
local at the site of infection. A
stronger stimulus leads to systemic
effects in the liver, brain and bone
marrow.
The results include increased white
cell production to help fight the
infection. Neurological effects such as
depression or sleepiness encourage the
animal to self-isolate, which in nature
would reduce the spread of disease –
in food-producing animals this
protective function cannot naturally
occur due to management factors.
Synthesis of acute phase proteins
is fuelled by amino acid release from
muscle, in turn reducing liveweight
gains. The acute phase proteins
perform a number of useful functions
including microbial inhibition,
coagulation and chemotaxis.
Neutrophil structure and
function
Neutrophils are important
components of the innate system and
are released from bone marrow as
short-lived cells, surviving only a
matter of hours. The cells contain
lysozyme granules that can kill bacteria
and glycogen granules, the cells being
able to be fuelled by anaerobic
glycolysis allowing them to enter low
oxygen areas of the body where there
are dead and dying tissues.
Neutrophils are known to roll
along blood vessel walls, loosely
adhering to endothelial cells and
moving as a result of the forces
imposed by vascular flow. Selectins
mediate this rolling action.
Integrins
allow adherence and once adhered,
neutrophils can digest the adhesions
between endothelial cells and squeeze through into infected tissue, sensing
the chemotactic gradient. Steroids
down-regulate expression of integrins
and reduce neutrophil adhesion. This
is one of the reasons that stress and
steroid administration can be
detrimental in infected animals.
Morphologically, neutrophils have
large membranes that facilitate
phagocytosis by wrapping around a
pathogen. Most bacteria repel this
membrane but antibodies opsonise
bacteria, making them easier to engulf.
Bacterial destruction involves both
oxidation and iodination. Professor
Roth noted that the neutrophil worked
out long before man that bleaching
and formaldehyde can kill bacteria.
The close interrelationship between
the innate and adaptive immune
system is also evidenced by the process
occurring during antibody dependent
cell-mediated cytotoxicity (ADCC)
which is mediated by natural killer
cells, neutrophils and macrophages.
In this process antibody binds
antigens on the target cell, receptors
on the killer cell recognise bound
antigen and cross link, in turn
triggering activation of the killer cell to
destroy the target.
A new mechanism described in
recent years is that of neutrophil
extracellular traps (or NETs). This is
thought to result in extracellular
killing, as a result of the neutrophil
digesting its nuclear material and
throwing out a sticky mixture of
DNA, histones and granular proteins
to engulf the pathogen.
NETs offer some prospects to
better understand immunity in the
periparturient period and Professor
Nahum Shpigel elaborated on
Professor Roth’s comments, describing
how high betahydroxybutyrate levels
(BHBA) are associated with reduced
NET’s functionality. Repeated bouts of
mastitis and metritis also suggest that
immunological “memory” plays little
part in the dairy cow’s defences with
regard to these conditions.
Neutrophil driven defences
Although neutrophils are a key
component of innate defences and
have a variety of ways in which they
exert their protective effect, the
functional capability can be affected by many factors.
Physiological states such
as the neonatal period,
high circulating
progesterone levels, stress
and the periparturient
period can all influence how neutrophils might fulfil their
protective function in bovine animals.
Up to five months of age, calves
experience reduced neutrophil
function both in terms of lower
iodination and lower ADCC responses.
As has already been noted, stress and
steroids can reduce neutrophil
adherence by as much as 25%; random
migration is increased but the ability
to adhere and leave the bloodstream is
reduced.
High progesterone levels produce a
very similar picture to high circulating
cortisol levels. In contrast, high
oestrogen-low progesterone levels are
associated with high levels of
neutrophil oxidation and iodination
and at this point the animal is
best positioned to respond effectively
to bacterial challenge.
During the periparturient phase,
neutrophil function dips at the point
of transition before beginning to
increase again post-parturition. Study
data have demonstrated that both IL8
production, chemotaxis of neutrophils
to cotyledon supernatant – a process
that normally aids expulsion of the
placenta – and myeloperoxidase
activity of neutrophils, were all lower
in cows experiencing retained placenta.
This suggests that immune
function, particularly neutrophil
function, is a critical factor in the
development of retained placenta.
Pathogens can also impact upon
neutrophil function. Bovine Viral
Diarrhoea (BVD) has been shown to
reduce neutrophil iodination and
administration of a modified live
vaccine of the same agent has the
same effect, reducing iodination by as
much as 75% of normal function – an
effect that can be compounded by also
administering cortisol at the same time.
Capsular material from some bacteria,
such as Pasteurella and Brucella can also
reduce neutrophil iodination.
Being able to increase neutrophil
function could clearly have significant
benefits for the periparturient cow in
particular. Cytokines, interferon
gamma and granulocyte colony stimulating factor (GCSF) are potential
candidates for this role. Interferon can
improve some aspects of neutrophil
function in immunosuppressed calves.
Ultra-filtered colostral whey has
also been shown to increase neutrophil
function in both cows and
periparturient cows. This may be
attributable to passive transfer of
cytokines, rather than antibodies
(which would be filtered out). GCSF
stimulates bone marrow to produce
neutrophils and daily administration to
periparturient cows has resulted in
increased numbers circulating in the
blood, reduced random migration and
increased ADCC and phagocytosis.
While antibody-led adaptive
immunity has previously been the
focus of veterinary intervention, it’s
clear that the innate immune system
plays an important role in the defences
of production animals and is
particularly important during the
periparturient period.
The role of neutrophils is key in
responding to infection and reduced
function and expression throughout
the vital 90 days means that the dairy
cow is vulnerable to developing related
conditions such as metritis, mastitis
and retained placenta.
- List of references available on
request to editor@veterinary-
practice.com.
