The gestation period of the mare is about 340 days. Lymphocytes are seen first in the thymus at about 60 to 80 days postconception. They are found in the mesenteric lymph node and intestinal lamina propria at 90 days and in the spleen at 175 days. Blood lymphocytes appear at about 120 days. A few plasma cells may be seen at 240 days. Graft-versus-host disease, a cell-mediated response, has developed in immunodeficient foals transplanted with tissues from a 79-day-old fetus. The equine fetus can respond to coliphage T2 at 200 days postconception and to Venezuelan equine encephalitis virus at 230 days. Newborn foals have detectable quantities of IgM and IgG and occasionally IgG3 in their serum, but IgE production in the horse does not begin until foals are 9 to 11 months of age. Like other large herbivores, the foal has a well-developed ileal Peyer’s patch that serves as a primary lymphoid organ and eventually involutes. Major B cell markers are expressed by 90 to 120 days gestation. IGHM and IGLC transcripts are expressed in liver, bone marrow, and spleen at all ages. The expression of essential B cell genes demonstrates that gene recombination and immunoglobulin class switching occur during equine fetal life. As a result, small amounts of IgM and IgG are detectable at birth. Despite this competence, B cell functions may be actively suppressed by regulatory T (Treg) cells during the first few months of a foal’s life.

Calf

Although the gestation period of the cow is 280 days, the fetal thymus is recognizable by 40 days postconception. The bone marrow and spleen appear at 55 days. Lymph nodes are found at 60 days, but Peyer’s patches do not appear until 175 days (Figure 21-1). Blood lymphocytes are seen in fetal calves by day 45, IgM+ B cells by day 59, and IgG+ B cells by day 135. The time of appearance of serum antibodies depends on the sensitivity of the techniques used. It is therefore no accident that the earliest detectable immune responses are those directed against viruses, using highly sensitive virus neutralization tests. Fetal calves have been reported to respond to rotavirus at 73 days, to parvovirus at 93 days, and to parainfluenza 3 virus at 120 days. Fetal blood lymphocytes can respond to mitogens between 75 and 80 days, but this ability is temporarily lost near the time of birth as a result of high serum steroid levels. T cell subpopulations are present in calves at levels comparable to adults, but B cell numbers increase significantly during the first 6 months after birth.

The Immune System and Intrauterine Infection

Although a fetus is not totally defenseless, it is less capable than an adult of combating infection. Its adaptive immune system is not fully functional; as a result, some infections may be mild or unapparent in the mother but severe or lethal in the fetus. Examples include bluetongue, infectious bovine rhinotracheitis [bovine herpesvirus 1 (BHV-1)], bovine viral diarrhea, rubella in humans, and toxoplasmosis. Fetal infections commonly trigger an immune response as shown by lymphoid hyperplasia and elevated immunoglobulin levels. For this reason the presence of any immunoglobulins in the serum of a newborn, unsuckled animal suggests infection in utero.

In general, the response to these viruses is determined by the state of immunological development of the fetus. For example, if live bluetongue virus vaccine, which is nonpathogenic for normal adult sheep, is given to pregnant ewes at 50 days postconception, it causes severe lesions in the nervous system of fetal lambs, including hydranencephaly and retinal dysplasia, whereas if it is given at 100 days postconception or to newborn lambs, only a mild inflammatory response is seen. Bluetongue vaccine virus given to fetal lambs between 50 and 70 days postconception may be isolated from lamb tissues for several weeks, but if given after 100 days, reisolation is not usually possible. Akabane virus acts in a similar fashion in lambs. If given before 30 to 36 days postconception, it causes congenital deformities. If given to older fetuses, it provokes antibody formation and is much less likely to cause malformations. Piglets that receive parvovirus before 55 days postconception will usually be aborted or stillborn. After 72 days, however, piglets will normally develop high levels of antibodies to the parvovirus and survive. Prenatal infection of calves with BHV-1 results in a fatal disease, in contrast to postnatal infections, which are relatively mild. The transition between these two types of infection occurs during the last month of pregnancy.

The effects of the timing of viral infection are well seen with bovine viral diarrhea virus (BVDV). If a cow is infected early in pregnancy (up to 50 days), she may abort. On the other hand, infections occurring between 50 and 120 days, before the fetus develops immune competence, lead to asymptomatic persistent infection because the calves develop tolerance to the virus (Figure 21-2). These calves are viremic yet, because of their tolerance, fail to make antibodies or T cells against the virus. Some of these calves may show minor neurologic problems and failure to thrive, but many are clinically normal. If the cow is infected with BVDV between 100 and 180 days postconception, calves may be born with severe malformations involving the central nervous system and eye, as well as jaw defects, atrophy, and growth retardation. Vaccines containing modified live BVDV may have a similar effect if administered at the same time. Calves infected after 150 to 180 days’ gestation are usually clinically normal.

Since they are specifically tolerant to BVDV, persistently infected calves shed large quantities of virus in body secretions and excretions and so act as a major source of infectious virus. The persistently infected calves may also produce neutralizing antibodies if immunized with a live BVDV vaccine of a serotype different from that of the persistent virus. Despite this, the original virus will persist in these animals. These persistently infected calves grow slowly and often die of opportunistic infections such as pneumonia before reaching adulthood. (BVDV has a tropism for lymphocytes and is immunosuppressive.) Their neutrophil phagocytic and bactericidal functions are also depressed.

BVD viruses occur in two distinct biotypes: cytopathic and noncytopathic. (The name derives from their behavior in cell culture, not their pathogenicity in animals.) Noncytopathic strains do not trigger type I interferon (IFN) production and therefore can survive in calves and cause persistent infections. Cytopathic strains induce IFN production and cannot cause persistent infection. These cytopathic strains, however, do cause mucosal disease (MD), a severe enteric disease leading to profuse diarrhea and death (Figure 21-3). Mucosal disease develops as a result of a mutation in a nonstructural viral gene that changes the BVDV biotype from noncytopathic to cytopathic while the animal fails to produce neutralizing antibodies or T cells. The cytopathic strain can spread between tolerant animals and lead to a severe mucosal disease outbreak. Both cytopathic and noncytopathic viruses can be isolated from these animals. Recombination may also occur between persistent noncytopathic strains and cytopathic strains in vaccines and lead to MD outbreaks. Although some of the lesions in MD are attributable to the direct pathogenic effects of BVDV, glomerulonephritis and other immune complex–mediated lesions also develop. The reasons for this are unclear but may reflect superinfection or the production of non-neutralizing antibodies. Because persistently infected calves can reach adulthood and breed, it is possible for BVD infection to persist indefinitely within carrier animals and their progeny. Epidemiological studies suggest that between 0.4% and 1.7% of cattle in the United States are persistently infected in this way.

Role of the Intestinal Microflora

The development of the newborn immune system is largely driven by the intestinal microflora (Chapter 22). In its absence, “germ-free” mammals fail to fully develop their mucosal lymphoid tissues. The commensal flora generates a complex mixture of pathogen-associated molecular patterns (PAMPs) that act through epithelial cell toll-like receptors (TLRs). Likewise, microbial antigens are taken up by dendritic cells and presented to CD4+ T cells. These signals collectively promote the functional development of the immune system. The intestinal microflora also plays a key role in determining any Th1 or Th2 bias in immune function. This is the basis of the “hygiene hypothesis,” the idea that the development of allergies is influenced by microbial exposure early in life (Chapter 28).

Innate Immunity

Newborns can produce a diverse array of antimicrobial molecules, including lectins such as the pentraxins and collectins, peptides such as the defensins, and lactoferrin and lysozyme. Surfactant proteins A and D as well as β-defensin 1 and TLR4 are produced in the preterm lamb lung. As a result, invaders can be killed relatively efficiently. TLRs are present and functional in the newborn. In the fetal pig, neutrophils at 90 days postconception are fully capable of phagocytosing bacteria such as Staphylococcus aureus. However, they are deficient in bactericidal activity, which only reaches adult levels 10 days later. Near birth, the phagocytic and bactericidal capacity of these neutrophils declines as a result of increased steroid levels. The neutrophils of newborn foals move relatively slowly compared with their dams. The serum of newborn mammals, however, is deficient in some complement components, resulting in a poor opsonic activity. Serum C3 increases rapidly after birth in newborn piglets and reaches adult levels by 14 days of age.

After birth, macrophages are able to support the growth of some viruses that macrophages from adult animals do not. Virucidal activity is gradually acquired. Changes also occur in the distribution of macrophages. Newborn piglets have few pulmonary intravascular macrophages. During the first few days after birth, blood monocytes adhere to the pulmonary capillary endothelium and differentiate into macrophages. In the newborn piglet, 75% of particles are removed from the blood by the liver and spleen, but by 2 months of age, 75% are removed by the lungs. The alveolar macrophages of newborn pigs have poor phagocytic activity, but this is effectively acquired by 7 days.

Newborn calves have fewer NK cells than adults, but these respond more strongly to simulation with interleukin-2 (IL-2) or IL-15 and are more cytotoxic! Age-dependent changes occur in the level of acute-phase proteins in newborn calves. Serum amyloid A (SAA), lipopolysaccharide-binding protein, haptoglobin and α₁-acid glycoprotein are present in high concentrations immediately after birth but gradually decline by 21 days. In developing piglets, IL-8 and tumor necrosis factor-α (TNF-α) production by blood monocytes increased significantly during the postnatal period, whereas monocyte production of IL-1β was unchanged.