Hereditary white blood cell disorders in the dog and cat

Phagocytes (neutrophils and macrophages) are involved in the innate immune response that is the first defence against non-specific bacteria and pathogens. Macrophages also orchestrate the specific response presenting the phagocytized antigens to T-helper (CD4_⁺) lymphocytes. The latter are the effectors of the specific immune response as they can activate the cell-mediated responses (Th1) producing cytokines that activate cytotoxic lymphocytes (CD8_⁺), natural killer (NK) cells and macrophages, or humoural responses (Th2) producing cytokines that activate the B-cells, which differentiate into antibody-producing plasma cells (IgG, IgA, IgM or IgE). The Th1 response is important in the destruction of cancer cells or cells infected by intracellular pathogens (e.g. protozoa), while the Th2 response is involved in the defences against viruses and pathogens of lesser entity, which after binding with the antibodies, activate the complement system or the cells responsible for antibody-dependent cytotoxicity.

The different leukocyte populations can be affected by hereditary diseases with different effects depending on the cell line involved. In general terms, the hypofunction of phagocytes leads to a greater exposure to non-specific pathogens, that of T-helper or B-cells leads to viral diseases and that of T-helper, cytotoxic or NK cells leads to diseases caused by intracellular pathogens.

MYELOID CELL ALTERATIONS

Hereditary abnormalities of phagocytosis
Domestic animals may be affected by hereditary abnormalities of the phagocytizing process, which is composed of 4 phases: adherence, chemotaxis, ingestion (in the strict sense of the word, phagocytosis) and digestion. Some diseases cause specific alterations of one or more phases of this process. The most common primary phagocytic disorders are:

Leukocyte Adhesion Deficiency (LAD): LAD is characterised by lesser adherence of the leukocytes (especially neutrophils) to the endothelium. The result is that the leukocytes are not able to leave the vessels and reach the site of phlogosis. Affected animals have recurrent localised (e.g. gingivitis, stomatitis, pneumonia, enteritis, mastitis, metritis) or systemic (septicaemia) infections, which respond to antibiotics but do not heal spontaneously. In the same animals a complete blood count (CBC) shows extreme neutrophilic leukocytosis since the continuous presence of pathogens stimulates the putting into circulation of neutrophils, which then are not able to leave the vessels.

There are at least two types of LAD. LAD-1, more widespread in animals, is an autosomal recessive disease in which, due to a mutation, the CD18 integrin - which once joined to the other integrin  CD11  allows neutrophils to adhere to the endothelium - is not present or is not able to bind to the CD11. LAD-2, on the other hand, is caused by a decreased expression of the selectins (involved in the rolling phase preceding adhesion) and/or of their ligands (e.g. Sialil-Lewis X).

The bovine form (Bovine Leukocyte Adhesion Deficiency or BLAD), caused by an amino acid substitution (glycine instead of aspartic acid in position 128) in the CD18, is associated with a decreased expression of selectins. In spite of the CD18 deficiency, neutrophils are still capable of reaching the pulmonary infection foci.

The canine form (Canine Leukocyte Adhesion Deficiency or CLAD), reported in the Irish Setter and caused by an amino acid substitution (serine instead of cysteine in position 36), leads to a decreased expression of CD18. Already as newborns, puppies often develop deadly infections and/or die in the early months of life.

The diagnostic suspicion arises from signalment and from the detection of persistent leukocytosis in the presence of equally persistent inflammatory processes. Diagnostic confirmation comes from the detection of the genetic abnormality through molecular tests developed in specialised laboratories. There is no specific treatment, even though both gene therapies and marrow transplants have been attempted experimentally, mainly to obtain comparative information on the treatment of the corresponding disease in humans. In the absence of specific treatments, symptomatic treatments have to be carried out in case of infectious diseases (antibiotics, support therapies). The possibility of identifying carriers through the above-mentioned genetic tests makes it possible to control the disease by excluding carriers from breeding programmes.

Chronic Granulomatous Disease (CGD): it is believed that a condition similar to this human disease exists in some canine breeds (e.g. Doberman). It is characterised by a series of mutations of the genes encoding NADPH oxidase, an enzyme that activates the cascade of oxidative events leading to the O₂-dependent digestion of pathogens (respiratory burst). Affected animals suffer recurrent infections that should be treated symptomatically.

Myeloperoxidase (MPO) deficiency: this is a common genetic alteration in humans, even though not very symptomatic because the neutrophils, though less efficient, manage to digest the ingested pathogens. Sporadic reports of forms similar to human MPO deficiency have been reported in dogs with widespread and recurrent infections (e.g. Grey Collies with cyclic haematopoiesis, Weimaraners).

Cyclic haematopoiesis in grey Collies: it is an autosomal recessive disease of Collies and Border Collies with coat colour dilution which cyclically manifest inflammatory forms caused by opportunistic bacteria associated with neutropenia, and to a lesser degree with lymphopenia, and with coagulopathies resulting from alteration of the platelet phase of haemostasis. Affected puppies usually die during the first year of life. The genetic abnormality resides in a mutation of the AP3 protein, the function of which is to transport proteins from the Golgi apparatus to the lysosomes. Transcription is blocked and the protein is not synthesised, given that AP3 is involved in the transport of elastase, which accumulates in the membranes and every 11-14 days causes haemopoiesis to stop.Neutropoiesis starts again after 2-3 days and neutrophilia can occur. The diagnosis is based on signalment, clinical history and possibly on the detection of low levels of myeloperoxidase, another protein often deficient in these patients. GM-CSF can be administered during the phases of neutropenia to normalise the neutrophil counts more quickly.

Chédiak – Higashi Syndrome: this is an autosomal recessive syndrome reported in some breeds of bovines and in the Persian cat, caused by a microtubular dysfunction which causes alterations in the motility and chemotaxis of neutrophils nand an abnormal fusion of phagosomes and lysosomes, followed by the presence of larger than normal granules, identifiable with  normal routine dyes. The abnormal fusion of the granules also leads to an accumulation of vacuoles inside many epithelial systems (e.g.: kidney), to abnormal platelet degranulation with resulting coagulopathies, as well as to “discolouration” of the pigmentation caused by the abnormal distribution of melanin granules. Affected subjects are thus more exposed to infectious diseases. It is therefore recommended to try to prevent infections or haemorrhages and/or to intervene early with symptomatic treatments.

Morphological abnormalities of granulocytes and monocytes
Some hereditary diseases cause cytoplasmic or nuclear alterations of the leukocytes, not associated with phagocytic disorders; such abnormalities are often only a part of more complex diseases affecting other systems of the body. Other diseases also exist in which the morphologically animal leukocytes are also not very functional (e.g. Chédiak-Higashi syndrome).

Pelger-Huet Anomaly: this is a hereditary disease that affects several animal species and several breeds of dogs (Australian Shepherds, Dobermans) in which the nuclei of the granulocytes do not mature; they remain hyposegmented but have normal chromatin condensation and function perfectly in dogs. The detection of hyposegmented neutrophils or eosinophils (Fig. 1) is incidental and should not be interpreted as a leukocyte left shift, usually detectable in dogs with inflammatory disease symptoms and/or with leukocytosis. The acquired form (pseudo Pelger-Huet) is associated with myelodysplasia. In the rare cases reported in cats this form is instead associated with functional abnormalities of the neutrophils and skeletal alterations. In humans the mutation concerns the gene encoding lamin, the protein involved in nuclear segmentation.

Abnormal neutrophil granulation in the Birman cat: an autosomal recessive condition in which neutrophils and their precursors contain fine eosinophilic granulations. Notwithstanding this pigmentation, neutrophils appear to function normally and affected cats are asymptomatic.

Grey eosinophils in the Greyhound dog: eosinophils with granules lacking a normal colour affinity (Fig. 2) have been reported in Greyhounds and, recently, in other hound breeds. In all of these breeds, up to 40% of individuals may present "grey" eosinophils, while in other breeds this is an occasional finding. All the eosinophils of affected subjects appear colourless. However, the constituents of the granules do not appear to be abnormal, to the extent that the granules that are released after artifactual cell rupture are normally coloured. The defect apparently consists in the inability to bind to normal dyes, perhaps due to a different cytoplasmic pH. Affected subjects do not show functional abnormalities of eosinophils.

Mucopolysaccharidosis: these are conditions caused by enzymatic defects in the catabolism of glycosaminoglycans (GAG) that make up mucopolysaccharides, so the latter cannot be degraded and tend to accumulate in the cells that contain them. Although granules can accumulate in more or less all tissues, the greatest changes are found in neutrophils and in cartilaginous cells. Large and intensely coloured granules are therefore found in both these cell types and should not be confused with Dohle bodies or signs of neutrophil toxicity. Depending on the mucopolysaccharides that accumulate, at least 7 forms of mucopolysaccharidosis (Table 1) have been identified. Neutrophil function does not appear to be impaired and the major symptoms are thus linked to the growth abnormalities associated with osteocartilaginous problems. Some forms of mucopolysaccharidosis can be diagnosed by determining the content and/or type of GAG in the urine in specialised laboratories.

Table 1. Main forms of mucopolysaccharidosis described in domestic animals

Diseases caused by lysosomal accumulation in the monocytes:the deficiency of enzymes involved in cell metabolism can lead to substrate accumulation in the lysosomes of the monocytes. Several diseases have been recognised based on the substrate present (Table 2). In the majority of cases, these abnormalities do not cause monocyte dysfunctions, even though potentially immune-suppressing cytopenias can be formed. The diseases in question are, however, also characterised by substrate accumulation in other cell systems, especially in the neurons, and thus these diseases are associated with neurological symptoms that are often incompatible with life.

Table 2. Main forms of diseases caused by lysosomal accumulation in the monocytes/macrophages described in domestic animals

ALTERATIONS IN THE LYMPHOCYTES

B lymphocyte abnormalities
Functional B cell disorders that lead to a deficiency in the production of antibodies are more often acquired (e.g. failed or delayed colostrum feeding). However, hereditary forms have also been reported, including:

Agammaglobulinaemia: reported in the male horse (probable sex-linked heritability, as occurs in humans, where it is caused by a tyrosine kinase deficiency which is important for B cell development), but not in the dog, in which, instead, there are self-limiting forms of transient hypogammaglobulinaemia, characterised by a reduced production of IgG and IgA, which returns to normal within 5-6 months. The clinical signs of these conditions appear after the decrease in maternal passive immunity (between 8 and 25 weeks), consisting in recurrent infections, lymphopenia (especially in B cells) and low or absent levels of IgA, IgG and/or IgM in the blood. Treatment consists in antibiotic treatments when infectious diseases develop. A particular form of hypogammaglobulinemia (IgM and IgA, more rarely also IgM) is observed in Shar Pei dogs, in which, however, the pathogenesis is probably more complex, as the disease appears at an advanced age; this condition also involves deficiencies in T cell function and is clinically characterised, apart from recurrent fevers and infections, by the appearance of intestinal tumours.

IgA deficiency: associated with a greater susceptibility to infectious diseases, especially due to greater mucosal permeability to pathogens. It is especially frequent in humans and can be total or partial; in some cases, it may be transient and disappear in the adult. In the dog, it was first described in Beagles and in Shar Peis and was then found in many other breeds (German Shepherds, Cocker Spaniels, Dobermans, Schnauzers and other breeds in which it however occurs sporadically). Some forms, in which the heritability has not been identified, are of differing severity in the dog, with a possible return to normal IgA values within 12-18 months. In cases of total IgA deficiency, recurrent though not especially severe respiratory diseases or staphylococcal dermatitises can be observed.The diagnosis is based on the quantification of serum IgAs, limited to some specialised laboratories, bearing in mind that in any case IgA values in puppies are lower than in adults. Treatment is based on the symptomatic treatment of any infectious diseases present.

IgM deficiency: reported in humans and in horses; it causes infectious diseases in various locations, which are usually bacterial, progressive and fatal. In the dog it is reported only sporadically, in the absence of other laboratory alterations (e.g. lymphocyte counts are normal). No treatment is available, apart from those aimed at the infectious diseases that develop in affected animals.

T lymphocyte abnormalities
Hypotrichosis and thymic aplasia in Birman cats: this is an autosomal recessive disease characterised by the birth of kittens without hair and thymus, which die in just a few days time. The genetic abnormality has not been identified, although it is believed to be the same as that of athymic nude mice.

Immunodeficiency associated with growth hormone (GH) deficiency: this is a condition reported in Weimaraners. The symptoms appear at 6-7 weeks of age and are characterised by a decline in general physical conditions and blocked growth, associated with persistent infections caused by a reduction in T lymphocyte activity. The concentration of immunoglobulins is instead normal. The administration of GH improves growth and favours an increase in thymus volume and function.

Acrodermatitis in Bull Terriers: this is a form of T cell hypofunction which is secondary to an autosomal recessive genetic disorder that concerns zinc metabolism. Affected animals mostly develop skin (pyoderma, acrodermatitis), intestinal and respiratory disorders, as well as stunted growth. Examination of the lymphoid organs shows hypoplasia of all T-dependent (especially thymic) areas. In spite of the fact that the pathogenesis resides in a zinc deficiency, the administration of zinc does not lead to clinical improvement and affected animals die quickly.

Combined immunodeficiency (severe combined immunodeficiency or SCID): this term actually includes several disorders having a different pathogenesis but all characterised by the suppression of both humoural and cell-mediated immunity. Affected subjects are incapable of developing an immune response and, as soon as the maternal passive immunity decreases, they exhibit symptoms caused by recurrent infections and to chronic and progressive disorders. In veterinary medicine, the following conditions are know:

SCID in Arabian horses: an autosomal recessive disorder characterised by severe lymphopenia and hypoplasia of lymphoid organs. It is caused by a mutation of the gene that encodes for a DNA-dependent protein kinase involved in the rearrangement of the T (TCR) and B (BCR) cell receptors, while natural killer (NK) cells are normal. The lack of the enzyme does not allow rearrangement, so  mature lymphocytes are not capable of recognising the antigens; their precursors are therefore eliminated before maturing. Once colostral immunity has disappeared, foals develop ingravescent mucopurulent respiratory conditions that lead to death during the first 5 months of life. At necropsy, mixed infections (Adenovirus, Rhodococcus equi, cryptosporidia) are detected in several organs. The diagnosis is based on the presence of severe lymphopenia and IgM deficiency, as well as on the necropsy findings. A genetic test is also available, which indentifies both affected animals and carriers. Given that no effective treatment exists, it is essential to set up prevention plans based on the exclusion of carriers from breeding programmes, using the above-mentioned genetic tests. A condition with a similar pathogenesis has been reported also in the dog, in a family of Jack Russell terriers.

Fell pony syndrome: a deadly disease of unclear heritability which manifests itself with the development of multiple chronic infections and a decline in overall conditions, associated with anaemias and severe lymphopenia and with severe necrotising inflammatory lesions in various organs. Affected foals have low IgM and IgA levels and a B-cell reduction in the blood and tissues.

X-linked SCID or XSCID: this is the most severe form, characterised by a decrease in T lymphocytes and circulating NK cells and by functional disorders of the B cells. It was identified in Basset Hounds, and later in Cardigan Welsh Corgis, but was then eradicated by excluding carriers from breeding programmes. The genetic defect resides in a mutation, which differs in the two breeds, in the gamma chain of the interleukin 2 receptor (IL-2R); this chain, which is no longer functional following the mutation, is also found in the receptor for other cytokines, so the lymphocytes are not affected by the mitogenic stimulus induced by the IL-2.

Affected puppies show thymic hypoplasia and greater susceptibility to viral and bacterial infections (cystitis, pyoderma, ear infections, multiple gastrointestinal infections), which affect their growth and cause death at a young age. Experimental studies have shown that marrow transplant and gene therapy, similarly to the approach used in humans, can be effective also in dogs.

Suggested readings

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