Immunity at Birth and the Immune Development of the Foal
Angela I. Bordin
Despite a lengthy development of the innate and adaptive immune systems in the equine fetus, the foal is born with a relatively immature and naive adaptive immune system.
In addition, the epitheliochorial placentation in the mare precludes transfer of immunoglobulins in utero, and foals are born hypogammaglobulinemic.1 At birth they rely heavily on colostral IgG for protection against infection but also on colostral soluble molecules and cells for stimulation of the immune system. In horses, colostrum absorption occurs within the first 24 to 36 hours after birth (with a peak around 8 hours) through specialized enterocytes that allow absorption of macromolecules. After this period, these specialized cells mature, and absorption of immunoglobulins ceases. Consumption of antibody-rich colostrum therefore is essential for protection against pathogens not only immediately after birth but also during the period until foals can produce their own antibodies at protective levels, which may require 2 to 3 months. Therefore in foals with low or inadequate transfer of colostral IgG, hypogammaglobulinemia increases the susceptibility to infections during the first few months of life. The knowledge of the actual (rather than an estimated) IgG concentration in the foal's blood after ingestion of colostrum could be used for clinical planning and monitoring of risk of infection in the initial months of life, and tests are repeated as needed during this phase. Besides antibodies, other factors are possibly transferred to the foal via colostrum, including the cytokines IFN-γ, IL-4, IL-6, and TNF-α.2-4 Maternal lymphocytes in colostrum can also be absorbed through the GI tract and participate in adaptive immune responses of the equine neonate. Equine colostrum contains primarily CD4+ and CD8+ T cells and few B cells. These cells can express inflammatory cytokines (IFN-γ and IL-17) upon stimulation.5There is evidence that the immune system of foals actively develops during the fetal period. For instance, the detection of Igs in the fetus and presuckle foal, despite their low levels, suggests antibody production in utero.6 Indeed, antigen-specific antibody production and antigen-specific IgG isotype switching have been measured in the equine vaccinated fetus or potentially in the foal born to naturally infected dams.7-9 In addition, functional lymphocytes and lymphoid tissue organization have been detected in the fetus by 100 (T cells) and 200 days (B cells), and presence of IL-8 in presuckle foals suggests endogenous production.2,10 Despite maternal antibodies, foals have been shown to produce endogenous serum IgM and IgG at protective values by 2 ± 3 months of age, regardless of colostrum ingestion.11-15 Although foals can make their own antibodies despite colostrum consumption, prepartum vaccination of mares is important because Ig levels in mares' serum correlate with levels in mammary secretions, and foals with inadequate transfer of passive immunity through colostrum are more susceptible to infections, particularly to those pathogens that cause septicemia and are challenging in the first few months after birth.16
The development of the immune system continues intensively after birth with the exposure to both pathogenic and nonpathogenic microorganisms in the environment. In individuals not exposed to microbes, such as germ-free mice and rabbits, both innate and adaptive immunity are not as well developed as those exposed to environmental microbes and present many immunologic defects.17 Although research of the microbiome of horses and foals is still a developing area, it has been demonstrated that it changes significantly with age.18-21 By 1 month of age, the microbiome of foals is significantly more diverse than at birth, and both changes and increases in bacterial communities in the gut likely reflect the replacement of milk diet by available forage and concentrate fed to the mare.18-21 It has been shown that diversification of the microbiome in neonates of all species influences various developmental processes such as immunity to infection.22,23
Absolute peripheral blood lymphocyte counts in healthy equine neonates are somewhat comparable to adult horses, but prematurity or immaturity is often characterized by lymphopenia.15,24 Therefore a rapid increase in the blood lymphocyte count (at least two times) is expected in the initial few months of life in a normally developing immune system responding to microorganisms.15 Hence both B and T lymphocyte subpopulations increase with age.12,15,25 In addition, B-cell markers such as B29A+ and CD21+, as well as MHC class II, also increase with age, whereas the proportion of CD8+ lymphocytes decrease with age.25
Innate immune cells are fundamental for protection against infections soon after birth and a crucial link between innate and adaptive immune systems.
In foals, phagocytic function by innate immune cells is particularly important for protection against disease because of the lack of developed adaptive immunity.26 Neutrophils are the most abundant effector cells of innate immunity in blood and the first line of defense by containing the infection via phagocytosis and subsequent killing of invading pathogens, as well as producing both cytokines and chemokines that recruit and activate other effector cells.27,28 In general, studies report comparable neutrophil function in foals and adult horses; however, some studies showed decreased bactericidal activity in individual foals relative to adults, including during sepsis.15,29-31 Phagocytosis and killing of yeasts by neutrophils increase with age, being lower in newborns before colostrum ingestion and reaching adult levels by 1 month of age.32-34 Neutrophil chemotaxis also increases with age.35 Macrophages are another important type of phagocytic cells: they ingest and kill intracellular pathogens and recruit other phagocytes to the site of infections; in addition, they function as APCs. In a study comparing newborn, 3-month-old foal, and adult alveolar macrophages and monocyte-derived macrophages, the authors observed a similar capacity of R. equi intracellular killing in newborn and adult horses.36Opsonization of pathogens with antibodies plays an essential role in their uptake by phagocytes through antibody Fc receptors and subsequent bactericidal capacity; therefore hypogammaglobulinemia markedly limits phagocytic activity and protection.29,30,32,37,38 The complement system is also important for the defense against pathogens, but colostrum is not a reliable source of this protein, and foals depend on their own production for protection.39 Complement activity in foals increases with age, being 13%, 64%, and 85% of that in adult horses in the first week after birth, at 1 month, and at 5.5 months of age, respectively.34
DCs are APCs to T lymphocytes for the generation of cell-mediated responses.
There are functional differences between DCs of adult horses and foals, in which cells from young foals are less able to produce cytokines in response to stimulation (e.g., lipopolysaccharide [LPS], cytosine-phosphate- guanosine oligodeoxynucleotide [CpG-ODN]) and are considered less mature than cells in adult horses.40 DCs from foals have reduced expression of MHC class II and co-stimulatory molecules, meaning that their potential for interaction and ability to stimulate naive T cells is limited.15,41 In addition, foals' APCs also express less CD1, which is a surface cell molecule involved in the presentation of lipid antigens to subsets of T cells, in comparison to adult horses' cells.42Under physiologic conditions, serum IgM concentration of 25 mg/dL but less than 20 mg/dL for IgG may be measured in presuckle foals.10,13 Nevertheless, endogenous synthesis of IgGb (IgG4/7) cannot be detected until 63 days old.43 IgGb (IgG4/7) is the most common isotype in serum of both adult horses and foals, as well as in colostrum, and IgA is the major isotype found in nasal secretions of adult horses and milk. In nasal secretions of foals, however, IgGa and IgGb are the major isotypes.43 Maternal antibodies decline within the first 3 months of age, with an IgM half-life of 6 ± 16 days and an IgG half-life of 28 ± 32 days.11,43 Serum half-lives of Ig isotypes in foals are 17.6, 32, 21, and 3.4 days for IgGa (IgG1), IgGb (IgG4/7), IgGT (TgG3∕5), and IgA, respectively.43 Yet the equine fetus and newborn can respond to vaccination with antibody production, indicating that the humoral response is in place at early life. Two important studies demonstrate that early intramuscular vaccination of newborns can elicit specific yet modest antibody 4445
responses to antigens.
, However, the development of a long-lasting immune response with memory cells is limited at this age. The current recommended age for vaccination of foals born from mares vaccinated in the prepartum period is 4 to 6 months for most vaccines with the exception of botulism, for which the first dose can be administered at 2 to 3 months of age (according to American Association of Equine Practitioners [AAEP] guidelines). This recommendation aims to especially avoid maternal antibody interference, assuming adequate transfer of passive immunity.CD4+ Th1 responses (or Th1-associated cytokine response) are thought to be essential for protection against intracellular pathogens, whereas Th2 responses with a shift to antibody production are thought to be important for extracellular pathogens and parasites. Foals are known to have reduced IFN-γ responses at birth.46,47 Although foal peripheral blood lymphocytes respond to nonspecific stimulation with a mitogen, foals produce less cytokine compared to adult horses, and this response increases with age in vivo and in vitro.48 Bronchial lymph node lymphocytes of foals produce lower levels of IL-4 (a hallmark for Th2 response) than those of adult horses.49
Despite being considered partially immature and naive, both innate and adaptive immune systems of foals can be stimulated to generate an immune response. Parapoxvirus ovis (PPVO) and inactivated Propionibacterium acnes (PA) were used as immunostimulants in foals: PPVO increased phagocytosis in neutrophils, and PA reduced intracellular replication of R. equi.50 In addition, due to the expression of TLR9 by foal neutrophils, CpG-ODNs have shown to stimulate cytokine as well as reactive oxygen species (ROS) production both in 275153
vitro and in vivo.27,51-53