Download Foal Immunity—Clinical Applications

HOW-TO SESSION:
LIFE STAGE MANAGEMENT
Foal Immunity—Clinical Applications
David W. Horohov, PhD
Although foals have a functional immune system at birth, several aspects of it are immature. This
leaves them susceptible to infectious agents. Enhancement of foal immunity can lessen the
effect of infectious disease. Author’s address: Maxwell Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546-0099; e-mail: [email protected]. © 2016 AAEP.
1.
Introduction
The fetal immune system develops in a sterile and
protected environment, and therefore lacks antigenic experience. Soon after birth, the newborn is
exposed to the “hostile world” of bacteria, viruses,
fungi, and parasites. The exposure to these microbes both challenges the health of neonates and
drives the maturation of their immune responses.
This initial immaturity of the immune response in
foals is considered to be responsible for their susceptibility to viral and bacterial infection, such as
Rhodococcus equi (R. equi). R. equi, remains one
of the most important causes of high-morbidity
respiratory disease in foals. As such, it is important to understand what part of the naivety of the
neonatal immune response is responsible for this
susceptibility given that this affects our efforts to
reduce the effect of this and other infectious
diseases.1
The immune response of the horse is composed of
both innate and adaptive systems to defend against
infections. A wide range of distinct cell types comprise the immune system, each of which plays an
important role in both innate and adaptive immune
responses. The lymphocytes engage in the central
role in adaptive immunity given that they are the
cells that determine the specificity of immunity-spe-
NOTES
448
2016 Ⲑ Vol. 62 Ⲑ AAEP PROCEEDINGS
cific antibody production, cytokine secretion, and cytotoxicity. It is their response that directs the
effector limbs of the immune response, humoral immune response, and cell-mediated immune response. Other types of cells interact with
lymphocytes either in the way of antigen presentation or mediation of immunologic functions. These
cells include: cells that eliminate invaders and initiate adaptive immunity during innate immunity
such as granulocytes (neutrophils, eosinophils, and
basophils), cells that present antigens to lymphocyte
such as dendritic cells and monocytes/macrophages.
These latter cells represent an important bridge between the innate and adaptive immune responses.
Although all of these cells are present in the neonatal foal, their functionality is impaired compared
with adult cells.
B Cells
Prior to birth, B cells are immunologically competent, but their production of antibodies is limited in
neonatal foals, although immunoglobulin production increases rapidly after birth.1 As such, passive
immunity transferred via colostrum is critical given
that immunoglobulins are rarely transferred to the
fetus due to epitheliochorial placentation. The relative concentration of immunoglobulins in the colostrum is such that IgG (IgGb⬎IgGa⬎IgGT) ⬎IgA
HOW-TO SESSION:
(negligible within 12–24 h) ⬎IgM.2 Neonatal absorption of immunoglobulin occurs within 2 hours of
birth and is mediated by unselective pinocytosis
through specialized enterocytes. The maternal immunoglobulins can be detected in the foal’s circulation within 4 – 6 hours and peaks at 18 –24 hours.
Thereafter, these maternal immunoglobulins decrease gradually in foals, and disappear after 1
month for IgA and IgM, and after 6 months for IgG
subtypes. Production of antibodies by foal B cells
can occur in utero and steadily increases following
birth. Very young foals can respond to vaccination,
although their ability it produce all sub-types of
antibodies remains impaired until 2 months of age.2
This age-related impairment of sub-isotype production is due, in part, to the immaturity of the T cell
response in young foals.
T Cells
Functional T lymphocytes are present in equine fetus by day 100 of gestation and are capable of responding to stimulation by day 140.3 After birth,
T-cell populations undergo expansion, similar to
that seen in B cells. The numerical increase in the
number of circulating T cells is primarily due to an
increase in the CD8⫹ T-cell population, given that
the proportion of CD8 T cells increases nearly 5-fold
by the fourth month of age, whereas CD4⫹ T cells
remain fairly constant with age.4 This selective
expansion is likely in response to the foal’s exposure
to environmental microbes.5 This change in T-cell
numbers is also associated with an alteration in
their functionality as evidenced by their production
of various cytokines. Cytokines are potent regulators of innate and adaptive immunity. The expression of key cytokines such as interferon
gamma (IFN-␥), IL-4, IL-17, and IL-10 represent
the functions of specific T-cell subsets including
type 1 helper T (Th1), type 2 helper T (Th2), type
17 helper T (Th17) cells, and regulatory T cells
(Treg), respectively.6,7 Decreased expression of
these cytokines by peripheral T cells in neonatal
foals compared with adult T cells suggests an
overall impairment of T-cell function in foals.
Among all of the cytokines, the reduced production
of IFN-␥, an indicator of cell-mediated immune
response, is considered an important contributor
to the foal’s susceptibility to viral and intracellular bacterial infections.8
Antigen-Presenting Cells
The activation of lymphocytes is primed and modulated by antigen-presenting cells (APC), which function as a bridge linking innate and adaptive immune
response. The APC engulf, process, and present
antigens to T cells. This antigen presentation to
T cells is mediated via surface molecules and the
secretion of various cytokines. This APC population is composed of monocytes, macrophages, and
dendritic cells, the latter being specialized and
highly efficient APC. Not surprisingly, the APC of
LIFE STAGE MANAGEMENT
young foals exhibited decreased expression of these
accessory molecules and cytokines.9 Here again,
the maturation of these cells into a more adult-like
phenotype seems to occur in response to stimulation
by environmental antigens.
Respiratory Immunity
The respiratory tract provides the second largest
route of exposure to invading microbes. Unfortunately, the immune system of the respiratory tract
in foals is functionally immature. In particular,
the mucosa-associated lymphoid tissue (MALT),
which plays an important role in the protection of
the respiratory tract, is incomplete in foals.10 The
number and competency of immune cells is impaired
in foals compared with that of adult horses. In
adult horses, MALT in the respiratory tract is composed of nasal-associated lymphoid tissue, pharyngeal tonsils, laryngeal (LALT), trachea-associated
lymphoid tissue, and bronchus-associated-lymphoid
tissue. The appearance of the MALT begins in the
fetus and gradually develops until 2 years of age.
The first appearance of isolated lymphoid nodules
occurs in the fetus as early as 9 months’ gestation at
the vestibule, nasal cavity, nasophaynx, and LALT.
The number of nodules shows a marked increase
after birth and reaches the adult level by 2 years of
age. The nasopharyngeal tonsil forms the largest
single mass of lymphoid tissue in the respiratory
tract. However, bronchus-associated-lymphoid tissue is not present in the fetus and neonatal foal, and
only found in older foals. In adult horses, organized lymphoid nodules and predominately unorganized infiltrates of closely packed lymphocytes are
seen in small intrapulmonary bronchi and these
structures are absent in the lungs of neonates.
Such structures typically begin to appear by 8 –22
weeks of life. This age-associated distribution of
mucosal lymphoid tissues reflects a gradual maturation of the respiratory immunity in foals. The
association of the occurrence of the nodules at specific sites within the tract and the areas where inhaled antigens accumulate suggests an influence of
environmental exposure on this development.
Bronchial Alveolar Lavage Cells
Not only does the distribution of respiratory lymphoid tissue in the foals exhibit an age-related development, but the maturity of the immune cells
within the lung also follows an age-dependent development. Besides being located in MALT, lymphocytes are distributed diffusely throughout the lung
in the walls of the airways, the mucus, parenchyma
tissues, and alveolar spaces. Current understanding of foal lymphocyte function in the lung is based
mostly on the analysis of bronchial alveolar lavage
(BAL) samples. Although lymphocytes compose
approximately 40% of the total BAL cells in adult
horses, they represent 4 – 6% of the cells in BAL
from neonatal foals, the remainder being macrophages and some granulocytes. Both the absolute
AAEP PROCEEDINGS Ⲑ Vol. 62 Ⲑ 2016
449
HOW-TO SESSION:
LIFE STAGE MANAGEMENT
number and the frequency of lymphocytes in BAL
fluid has been shown to increase over time. B cells
in particular are initially virtually absent at birth,
although IgG-, IgM-, and IgA-producing plasma
cells appear 1 week after birth and the numbers of
the cells reaches an adult level in foals by 12 weeks
of age. A reduced number of T cells in the BAL is
also seen in foals less than 6 weeks old. This reduced number of lymphocytes present in alveoli and
the lack of available lymphocytes to participate in
the immune response in foals less than 6 weeks old
may have relevance for foals’ susceptibility to pulmonary infection. An impaired function of lung
T cells is also evident in neonatal foals as they
exhibit low expression of cytokine, particularly
IFN␥. However, over time there is increased expression of this cytokine likely as the result of exposure
to environmental antigens.5 Together, these deficiencies likely provide the opportunity for infections
to occur.
Environmental exposure to antigens likely drives
the maturation of the immune system. This postnatal exposure leads to both the recruitment of cells
into secondary lymphoid sites and can play a role in
directing the immune response toward a specific
cytokine response. Thus, early exposure to bacterial antigens is thought to favor the induction of
Th1-type immunity and prevents the development
of allergic and autoimmune diseases associated
with Th2 immune responses. As such, the microbiome of the respiratory and gastrointestinal systems plays a key role in the overall development of
immune competency in the neonate. Perturbations of these biomes may lead to alterations in
subsequent immune responses favoring Th2 over
Th1 responses. This contribution of the microbiome in influencing immune development likely
represents the underlying mechanism of the “hygiene hypothesis.”11 However, a recent study
found no association between the gastrointestinal
microbiome of healthy foals and those infected
with Rhodococcus equi.12
2.
Clinical Application
Given the immaturity of the foal’s immune system
at birth, several steps can be taken to maximize its
effectiveness. Maternal antibodies are an essential
component of the initial immune repertoire of the
foal.13 The importance of passive transfer and the
need for the foal to acquire sufficient maternal antibodies cannot be overstated. A post-suckling
plasma immunoglobulin concentration of 800 mg/dL
should be considered the minimum acceptable
level. The quality of maternal colostrum is another
important consideration, referring to the amount of
antibodies present to specific agents. A mare vaccination program, as recommended by the American
Association of Equine Practitioners (www.aaep.org),
should be followed to insure the presence of adequate amounts of specific antibodies in the colostrum. Natural exposure of mares to pathogens in
450
2016 Ⲑ Vol. 62 Ⲑ AAEP PROCEEDINGS
the environment can likewise augment her ability to
provide the foal with protection. Earlier vaccination of foals that have received insufficient passive
transfer or whose mares were not adequately vaccinated should be considered. Although maternal
antibodies may inhibit responses to some vaccines,
there is little evidence to suggest a long-term negative effect.14,15 As such, it is better to vaccinate the
foal earlier rather than risk the possibility of it being
unprotected.15 In the case of R. equi, where no vaccine is available, the use of Rhodococcal-specific hyperimmune plasma should be considered for foals on
endemic farms. Although this treatment will not
prevent infection, a single treatment administered
after birth can reduce the severity of clinical signs in
foals that become infected.16 Variability in the apparent efficacy of hyperimmune plasma (HIP) in the
field may be due in part to differences in obtained
antibody levels obtained in treated foals.17 The reasons for this variability are unknown, although variations between products and different lots within a
product line does occur.17 Other methods of nonspecifically stimulating T-cell immunity in foals will
likely be unsuccessful until the foal reaches at least
1 month of age based on recent studies.18 The overall effectiveness of this approach in reducing R. equi
infections in the young foal remains uncertain.19
Likewise, the effectiveness of these treatments in
re-directing foal immune responses is also unknown.
Acknowledgments
The contributions of Drs. Lingshuang Sun and Macarena Sanz are gratefully acknowledged. Financial support for much of the work cited in this paper
was provided by the Grayson Jockey Club Research
Foundation, Bioniche, and the U.S. Department of
Agriculture. Additional support was provided by
the William Robert Mills endowment in the Department of Veterinary Science at the University
of Kentucky.
Declaration of Ethics
The Author has adhered to the Principle of the Veterinary Medical Ethics of the AVMA.
Conflict of Interest
The Author has provided paid consultation on immunology-related topics to a number of pharmaceutical companies and other related businesses.
References
1. Perkins GA, Wagner B. The development of equine immunity: Current knowledge on immunology in the young horse.
Equine Vet J 2015;47:267–274.
2. Sheoran AS, Timoney JF, Holmes MA, et al. Immunoglobulin isotypes in sera and nasal mucosal secretions and their
neonatal transfer and distribution in horses. Am J Vet Res
2000;61:1099 –1105.
3. Perryman LE, Wyatt CR, Magnuson NS, et al. T lymphocyte development and maturation in horses. Anim Genet
1988;19:343–348.
4. Flaminio MJ, Rush BR, Davis EG, et al. Characterization
of peripheral blood and pulmonary leukocyte function in
HOW-TO SESSION:
5.
6.
7.
8.
9.
10.
11.
12.
healthy foals. Vet Immunol Immunopathol 2000;73:267–
285.
Sun L, Adams AA, Page AE, et al. The effect of environment
on interferon-gamma production in neonatal foals. Vet Immunol Immunopathol 2011;143:170 –175.
Wagner B, Burton A, Ainsworth D. Interferon-gamma, interleukin-4 and interleukin-10 production by T helper cells
reveals intact Th1 and regulatory T(R)1 cell activation and a
delay of the Th2 cell response in equine neonates and foals.
Vet Res 2010;41:47.
Boyd NK, Cohen ND, Lim WS, et al. Temporal changes in
cytokine expression of foals during the first month of
life. Vet Immunol Immunopathol 2003;92:75– 85.
Breathnach CC, Sturgill-Wright T, Stiltner JL, et al. Foals
are interferon gamma-deficient at birth. Vet Immunol Immunopathol 2006;112:199 –209.
Mérant C, Breathnach CC, Kohler K, et al. Young foal and
adult horse monocyte-derived dendritic cells differ by their
degree of phenotypic maturity. Vet Immunol Immunopathol
2009;131:1– 8.
Mair T, Batten E, Stokes C, Bourne F. The distribution of
mucosal lymphoid nodules in the equine respiratory tract.
J Comp Pathol 1988;99:159 –168.
Daley D. The evolution of the hygiene hypothesis: The role
of early-life exposures to viruses and microbes and their
relationship to asthma and allergic diseases. Curr Opin
Allergy Clin Immunol 2014;14:390 –396.
Whitfield-Cargile CM, Cohen ND, Suchodolski J, et al.
Composition and diversity of the fecal microbiome and inferred fecal metagenome does not predict subsequent pneu-
13.
14.
15.
16.
17.
18.
19.
LIFE STAGE MANAGEMENT
monia caused by Rhodococcus equi in foals. PLoS One 2015;
10:e0136586.
LeBlanc MM, Tran T, Baldwin JL, et al. Factors that influence passive transfer of immunoglobulins in foals. J Am Vet
Med Assoc 1992;200:179 –183.
Sturgill TL, Horohov DW. Vaccination response of young
foals to keyhole limpet hemocyanin: Evidence of effective
priming in the presence of maternal antibodies. J Equine
Vet Sci 2010;30:359 –364.
Davis EG, Bello NM, Bryan AJ, et al. Characterisation of
immune responses in healthy foals when a multivalent vaccine protocol was initiated at age 90 or 180 days. Equine Vet
J 2015;47:667– 674.
Sanz MG, Loynachan A, Horohov DW. Rhodococcus equi
hyperimmune plasma decreases pneumonia severity after a
randomised experimental challenge of neonatal foals. Vet
Rec 2016;178:261.
Sanz MG, Oliveira AF, Page A, et al. Administration of
commercial Rhodococcus equi specific hyperimmune plasma
results in variable amounts of IgG against pathogenic bacteria in foals. Vet Rec 2014;175:485.
Sturgill TL, Strong D, Rashid C, et al. Effect of Propionibacterium acnes– containing immunostimulant on interferongamma (IFN␥) production in the neonatal foal. Vet
Immunol Immunopathol 2011;141:124 –127.
Sturgill TL, Giguere S, Franklin RP, et al. Effects of inactivated parapoxvirus ovis on the cumulative incidence of
pneumonia and cytokine secretion in foals on a farm with
endemic infections caused by Rhodococcus equi. Vet Immunol Immunop 2011;140:237–243.
AAEP PROCEEDINGS Ⲑ Vol. 62 Ⲑ 2016
451