CHAPTER 14 Immunologic Development and Immunization
The immune response is essential for survival from the wide array of infectious microorganisms to which the puppy and kitten are exposed. The immune responses of neonates are inferior to those of mature dogs and cats, not because components of the immune system are lacking, but because the soluble mediators are present in suboptimal concentration and cellular elements are in a naive state.
The transition from a protected environment in the uterus to an environment containing a variety of potential infectious agents requires a rapid response by the immune system to protect the neonate from infections. Many factors play a role in the newborn’s survival, including its innate immune system, acquired immune system, and passive transfer of maternal antibodies. Other factors, such as the dam or queen’s health, nutrition status, immunization status, parasitic control, breeding management, and environment, also have an impact on health, sickness, or survival of the neonate. Understanding immunologic development of the newborn is necessary to recognize the role of immune protection from birth to 12 months, the role of maternal antibody in early immune protection, as well as the response and effect of vaccination at an early age.
In general, innate immunity is a rapid, nonspecific first line of defense. Neutrophils, macrophages, and natural killer lymphocytes are the first responders, whereas cellular products such as complement and cytokines also play a role in innate immunity. Barriers, such as skin and mucosa, and normal bacterial flora are other components of innate immunity. Acquired immunity is a slower, but a specific second line of defense involving B lymphocytes, T-helper (Th) lymphocytes (CD4+ cells), and T cytotoxic lymphocytes (CD8+ cells). Acquired immunity is further broken down into humoral immunity (e.g., Th2 immunity) and cell-mediated immunity (e.g., Th1 immunity). Humoral immunity is typically directed against antigens that survive extracellularly, such as bacteria, protozoa, or fungal organisms, and relies on interactions between B lymphocytes and a subset of T helper lymphocytes, resulting in antibody secretion. Secreted antibody binds antigens, flagging them for destruction. The second part of acquired immunity is cell-mediated immunity, which involves activation of macrophages, natural killer cells, and antigen-specific T cytotoxic lymphocytes, which are the primary effector cell. The function of cell-mediated immunity is to destroy obligate intracellular organisms such as viruses, some protozoa, and bacteria. If neonatal puppies and kittens ingest colostrum, passive immunity, consisting of maternal immunoglobulins, will also aid in early immune protection. Puppy and kitten immunity depends on all parts of the immune system, although the function of each system varies with age and colostrum ingestion.
Development and function of complement pathways, antigen-presenting cells (APCs), phagocytic cells, and other pathways of the innate immune system have not been well characterized in puppies. However, one study observed increased phagocytic activity of neutrophils and macrophages in neonatal puppies compared to young adult dogs, suggesting there may be adequate function of phagocytic cells in puppies to help with the first line of defense against potential pathogens. In humans and mice, APCs have reduced ligand expression, which is necessary for stimulating specific immune responses, compared to adults. Although this has not been specifically evaluated in dogs, APCs may not be mature in neonatal puppies.
Immunologic development of the lymphoid system is incremental in all species. As a generality, the shorter the gestation, the less developed the immune system is at birth. In puppies, immunologic development is an early event, beginning in midgestation with lymphocytes present in circulation at 25 days of gestation (Figure 14-1). Thymic development begins on day 27 of gestation, followed by lymphoid infiltration into secondary lymphoid organs, including spleen and lymph nodes, on days 45 to 52 of gestation. Thymic development appears histologically normal by day 45 of gestation. In contrast, splenic and lymph node architecture is not completely developed in the fetus and lacks germinal centers and B cell follicles until shortly after birth.
At birth the numbers of peripheral lymphocytes increase and are initially composed of primarily B cells and CD4+ T cells with lower numbers of CD8+ T cytotoxic lymphocytes. The majority of T cells, almost 90%, are naive at birth compared to only 40% at 4 months of age, indicating progressive maturation of the T cell population during this time. The number of B lymphocytes in neonates is much higher than adults, but after the initial increase, B cell lymphocyte numbers decline until 16 weeks of age. The high proportion of B lymphocytes in newborns likely represents early B cell stimulation and maturation in response to new antigens. T cytotoxic lymphocytes are important in cell-mediated immunity and are low at birth. This finding may lend support to human and mouse studies showing downregulation of cell-mediated immunity in neonates. Because T cytotoxic lymphocytes are important for detecting and inactivating intracellular pathogens, the low numbers of T cytotoxic lymphocytes at birth may predispose a newborn to intracellular viral or bacterial infections. T cytotoxic lymphocytes increase steadily with age, whereas CD4+ T lymphocytes maintain at relatively steady numbers from birth to adulthood.
Knowledge of the distribution of peripheral lymphocytes is important to help determine if there are areas of immune deficiency in the neonate; however, function is another defining characteristic. Functional lymphocytes have been observed in the fetus, which is able to respond to antigens after lymphoid tissue develops in the last third of gestation. However, the antibody responses are variable and not as pronounced when compared to adults. Functional and specific lymphocyte responses are also present at birth, but similar to the fetus, antibody responses are often only a portion of the adult antibody response. One study observed adequate antibody response in day-old puppies immunized with a modified live canine parvovirus strain, suggesting some antigens may elicit an adequate antibody response, or some puppies have a higher level of immune development. In general, it is recognized that domestic animals are immunocompetent at birth; however, complete maturation of the immune system occurs postnatally.
Development of the kitten’s first line of defense, the innate immune system, including complement pathways, APCs, and other pathways of the innate immune system has not been well described. Neutrophil function has been evaluated, and phagocytic function is present at birth to help combat initial potential pathogens but is only a portion of adult response. The observed phagocytic response is independent of colostrum ingestion and matures to an adult response by 8 weeks of age. Based on human and mouse studies, it is speculated that APCs may also have reduced function in kittens; however, this hypothesis requires further evaluation.
Unlike the dog, there are only a few studies published discussing lymphoid development from the fetus to six months of age in kittens. Similar to the dog, lymphocytes have been detected in fetal circulation at approximately 25 days of gestation. Lymphocytes have also been detected in the thymus, spleen, and liver at between 28 to 52 days of gestation supporting fetal development of lymphoid tissue. In late gestation, there is a significant increase in peripheral T lymphocytes, which remain elevated after birth becoming the primary circulating lymphocyte in the adult. CD4+ T lymphocytes are present in higher numbers than CD8+ T lymphocytes, resulting in a high CD4:CD8 ratio at birth. The low numbers of CD8+ T lymphocytes at birth may be an effect of the queen’s immune status during pregnancy downregulating cell-mediated immunity, although this has not been evaluated in cats. CD8+ T lymphocytes begin to increase through 8 weeks of age, lowering the CD4+:CD8+ ratio. B cell numbers increase right after birth until 4 weeks of age, likely representative of maturation of the humoral immune response, then steadily decrease to adult values. The timeline for reaching adult distribution of lymphocyte subsets has not been definitively established but is thought to occur before 12 months of age. Similar to the puppy, kittens are immunocompetent at birth but appear to have restricted immune responses compared to adults. Antigenic stimulation of the fetus has not been documented in kittens so conclusions about fetal response to antigens are unknown.
The areas of maternal immunity pertaining to pregnancy, fetal survival, and postpartum periods have been the subject of active investigation. In its broadest context, the immune response of the pregnant dam and queen becomes compromised as the fetus matures. Although this immunodeficient state is transitory, it is now considered to extend through the postpartum period for up to 4 weeks.
The cellular mechanisms responsible for the pregnancy and postpartum immunosuppression are considered to revolve around two key cell populations. These involve the shift of T-helper cells from a Th1 to a Th2 response and a decrease in neutrophil functions. The T cell populations are affected by progesterone, prostaglandin F2α, and α-fetoprotein. The Th1 cells are important effectors of the cell-mediated immune (CMI) response and interact with T cytotoxic lymphocytes. As mentioned earlier, the T cytotoxic lymphocytes are the main defense against intracellular pathogens. With the onset of pregnancy, the hormonal factors cause macrophages to release predominantly Th2-stimulating cytokines that contribute to the overall dominance of humoral immunity during pregnancy and immediately postpartum. This phenomenon of the Th cell populations is referred to as the “Th1-Th2 shift of pregnancy” and is generally regarded as a contributing factor to maternal tolerance of the fetus by suppressing the antifetal CMI response.
The second key cell population affected during pregnancy and in the postpartum period is the neutrophil. The point of maximum immunosuppression occurs in the latter stages of pregnancy when there is an acute elevation in glucocorticoids. Neutrophil dysfunction and the effects on the Th cell population are considered temporary during this period. Nonetheless, with impaired neutrophil response, the animal is now vulnerable to increased bacterial infections caused by the compromised bactericidal functions.
The outcomes of the temporary immunodeficiency states allow for fetal survival but may also result in an increased susceptibility to environmental infections by bacteria and fungi. Intracellular infections, such as viruses and protozoans, which may have been acquired during postnatal development, may become exacerbated during pregnancy because of the suppressive effects on the Th1 cells. This suppression results in a decreased CMI response. The CMI effector cells, the T cytotoxic lymphocytes, function normally to control virtually all the viral infections such as canine herpesvirus. Intracellular bacterial infections, such as Brucella canis, become more pathogenic during pregnancy. In addition to the aforementioned effects on T cell function, macrophage function is also compromised, allowing opportunistic bacteria, which are usually confined to external mucosal surfaces of the body, to become systemic. Concurrent with the immunosuppression that accompanies pregnancy, there is an increased shedding of infectious microorganisms. This is considered to be an extension of impaired T cytotoxic lymphocyte function. Although the pregnant animal may appear clinically normal, her altered immune response results in an increased shedding of gastrointestinal viruses, such as canine coronavirus and canine rotavirus, and bacteria, such as Escherichia coli, usually during the periparturient period. This increase in shedding of infectious microorganisms is an important factor when addressing the issues of management of animals through this period of time (Figure 14-2). The suppression of neutrophil functions during later stages of pregnancy and immediately postpartum are the subject of active investigation.
Figure 14-2 Diagram depicts the three possible ways that neonatal puppies can be naturally infected. The dam infects the puppies before whelping or shortly thereafter (1); the dam acquires infection after whelping and infects the puppies (2); and the puppies are infected from an external source such as another dog (3).
Although the immunosuppressive periods during pregnancy and up to 4 weeks postpartum is well recognized, our understanding of this process is still somewhat rudimentary. However, we can proceed with control measures that accomplish two primary goals: to maximize reproductive performance and to assure successful neonatal survival. Over the years, we have emphasized the importance of effective vaccination programs prebreeding, clean birthing areas, and good hygiene for the lactating animal. In conjunction with good colostral management, these measures allow us to compensate for the temporary immunosuppressive states encountered during pregnancy and immediately thereafter.