This presentation reviews aspects of the development and maturation of the canine and feline immune system in the in utero and neonatal period and the immunological disorders that are of importance during early life.

In utero Development of the Canine and Feline Immune System

There is only rudimentary knowledge of the in utero development of the immune system in small companion animals. In the dog, development of the thymus commences on day 27 of gestation and is complete by day 45. Lymphocytes appear in the thymus on day 35, in lymph nodes on day 46 and in the spleen around day 50-55. Canine fetal splenic and thymic lymphocytes can be stimulated by mitogen from day 45 and 50 respectively. Lymphocytes appear in the circulation of fetal cats at approximately day 25 of gestation. In the final two weeks of gestation, the proportions of blood lymphocytes in the fetal kitten alters; with a marked elevation in T cells and reduction in 'null cells'. In the dog immune responses to a range of antigens are possible in the final trimester of gestation.

Maternally-Derived Antibody

The dog and cat have endotheliochorial placentation in which there is a relatively impenetrable barrier to the in utero transfer of maternal immunoglobulin. Small quantities of IgG may pass through this barrier, such that the newborn pup or kitten has a serum concentration of IgG that approximates to five per cent of the adult level.

In the first 24 hours after birth, the pup or kitten must ingest colostrum which provides passively acquired immune protection throughout the neonatal period. In newborn kittens, such absorption does not occur after the first 16 hours of life. It is assumed that this transfer involves the transient expression of the intestinal Fc R_n immunoglobulin receptor allowing absorption of IgG into the neonatal vascular and lymphatic circulations. IgM and IgA are also absorbed. There may be considerable variation between littermates in the efficiency of uptake of colostral immunoglobulin; relating to the size and strength of the individual newborn and the maternal abilities of the bitch or queen. There may also be variation between the individual bitch/queen in the concentration of specific antibodies within the colostral immunoglobulin.

Several studies have examined the immunoglobulin composition of canine and feline colostrum and milk. Canine colostrum is rich in both IgG and IgA and both immunoglobulins are present in higher concentration than in the serum of the bitch. By contrast, milk contains significantly more IgA than IgG and this IgA is also present in greater concentration than in canine serum. Newborn puppies have a serum IgG concentration of 1.2 mg/ml which increases to 23 mg/ml twelve hours after ingestion of colostrum. At the same time the concentration of serum IgA is 0.45 mg/ml and IgM 0.2 mg/ml.

Feline colostrum is also rich in both IgG and IgA, and both immunoglobulins are present in concentration greater than in the serum of the queen. By contrast to the dog, feline milk continues to be dominated by IgG. There is debate as to the relative concentration of IgG in feline colostrum and milk; early studies suggested that the cat did not have a specific colostral phase, with similar IgG concentrations in colostrum and milk but more recently, reduced concentrations of IgG and IgA have been demonstrated in milk relative to colostrum. After ingestion of colostrum, total serum IgG and IgA levels in the kitten peak and then gradually decline. Kittens may then have undetectable serum IgA between weeks one and six of life. Endogenous IgG production starts by 5-6 weeks of age and IgA production shortly after. By contrast, total serum IgM concentration in newborn kittens steadily increases to plateau at about day 60 of life.

Studies in other species have shown that in addition to immunoglobulin, maternal complement, leucocytes and cytokines may be absorbed from the colostrum. No published studies have yet addressed whether this occurs in pups and kittens. The uptake of maternally-derived immunity is an essential process, the failure of which leads to neonatal infection and death, but on the other hand, maternal immunoglobulin inhibits development of the endogenous neonatal immune response. The reported half-life for maternal IgG is approximately eight days in pups, but only 4.4 days in kittens. In the absence of passive transfer of maternal immunity, newborn pups are able to respond to antigen (e.g., parvovirus vaccine) as early as two weeks of age. Day old pups lacking MDA when vaccinated with modified-live parvovirus make a serological response 21 to 91 days post-vaccination similar in magnitude to that in older vaccinated puppies.

The point at which a newborn pup or kitten becomes immune competent is therefore variable and generally considered to be somewhere between 6-12 weeks of age. This knowledge underpins current vaccination protocols that advise three immunizations (e.g., at 8, 12 and 16 weeks) to be followed by a booster 12 months after administration of the final in this early life series.

Maturation of the Neonatal Immune System

The concentration of serum immunoglobulins (IgG, IgM and IgA) do not reach full adult levels until 12 months of age but few published studies provide such data. One study reports that serum IgG, IgM and IgA concentrations in 10 month old beagles are less than those in older dogs.

The blood lymphocyte count of dogs increases over the first week of life whilst neutrophil numbers decline. During the first three months of life, pups have higher blood lymphocyte counts than adult dogs but proportionally more of these cells are CD21⁺ B cells. These B cells decline in number to 16 weeks of age. Although the percentage of blood CD4⁺ T cells remains relatively stable from birth to adulthood in dogs, the percentage of CD8⁺ T lymphocytes is low at birth (thus causing a high CD4:CD8 ratio) and increases with age.

Similar studies of blood lymphocyte subsets have been performed for specific pathogen-free cats between birth and 90 days of age. Total blood lymphocyte count increases over this period with the most marked elevation being in B cells and CD8⁺ T cells, leading to a reduction in CD4:CD8 ratio as described for the dog. However, in contrast to the dog, the percentage of CD4⁺ T cells also increases over the same time period.

The thymus involutes during the first year in both species. In the cat this commences at 6-8 months of age and in the dog there is progressive decline between 6 and 23 months of age. Some studies have investigated developmental aspects of mucosal immunity in the dog. Physiological and immunological changes occur within the gastrointestinal tract during maturation. During the ingestion of colostrum, the canine small intestinal villi increase in size due to hypertrophy of enterocytes with cytoplasmic vacuolation and dilation of the lacteals. These changes are less prominent in the feline intestine 24 hours after birth.

Adult dogs have more T cells, plasma cells and dendritic cells at all levels of the respiratory tract than puppies, but puppies have significantly more mast cells and macrophages within the mucosa. There are no significant differences in transcription of genes encoding a range of cytokines and chemokines between puppies and adult dogs. Another study has reported concentrations of IgG, IgM and IgA in nasal secretions from puppies from birth to six weeks of age. IgG is the dominant class of immunoglobulin with the exception of time points between weeks one and three, where IgA was predominant. Significant variation was noted between animals and dogs of different litters.

A recent study has investigated gestational and early life canine immunity in the context of Toxocara canis infection. In this project, blood samples were taken from infected and control bitches throughout gestation and from their pups between four and 10 weeks of age. Blood lymphocytes were stimulated with mitogen or parasite antigen and the production of IL-10 and IFN-γ cytokines within these cultures determined by enzyme linked immunosorbent assay (ELISA). Cells from both infected and uninfected bitches produced increasing concentrations of IL-10 throughout the eight weeks of monitoring, with higher levels generated in cultures from infected animals. By contrast, IFN-γ production decreased throughout pregnancy. These data suggest that the immune system of the pregnant bitch is dominated by regulatory T cells and that the function of these cells can be amplified by concurrent parasitism. After birth, both infected and uninfected puppies produce IL-10 in culture, but the concentration of this cytokine declines to 10 weeks of age, whereas IFN-γ production increases. These observations suggest a switch from an immune system dominated by Th2/Treg to Th1 cells with increasing antigenic exposure.

Primary Immunodeficiency Disease

The range of genetic primary immunodeficiency diseases that affect dogs will often become clinically apparent following the waning of maternal immunity, although for some of the more subtle immunodeficiency disorders onset may be delayed until around 12 months of age. With few exceptions (e.g., the canine leucocyte adhesion deficiency, cyclic hematopoiesis, X-linked severe combined immunodeficiency, C3 deficiency) the precise genetic mutations responsible for the canine immunodeficiency disorders have not been characterized. In contrast to dogs, primary congenital immunodeficiency appears very rare in the cat.

Neonatal Isoerythrolysis

Neonatal isoerythrolysis (NI) is more common in the cat where it may be an important cause of the 'fading kitten syndrome'. NI is most likely to occur when a queen of blood group B is mated to a type A or AB tom and produces kittens of blood group A. As type B cats generally have high-titred anti-A alloantibodies these concentrate in the colostrum and are absorbed by the kittens leading to subsequent hemolysis (immune-mediated haemolytic anaemia). Affected kittens may become severely anemic, jaundiced and hemoglobinuric within a few days of birth. The kittens will be Coombs test positive. They may be weak and lethargic, fail to suckle and die. A subclinical form of the disease is also recognized in which kittens develop tail tip necrosis at 1-3 weeks of age. The prevalence of NI will be greater in those breeds that more commonly have cats of blood group B (e.g., Birman, Rex, British Short Hair, Somali, Persian and Abyssinian).

NI is uncommon in the dog, but can occur when a DEA1^- bitch is sensitized to DEA1⁺ blood following an incompatible blood transfusion given before the pregnancy.

References

References are available upon request.