JPC SYSTEMIC PATHOLOGY

HEMOLYMPHATIC SYSTEM

February 2024

H-M02

 

Signalment (86-6580): 7-week-old male Arabian foal.

 

HISTORY: The foal was normal at birth, but developed respiratory disease at one week of age.

 

HISTOPATHOLOGIC DESCRIPTION: Spleen: Diffusely, there is a paucity of white pulp. The periarteriolar lymphatic sheaths (PALS) are devoid of resident T-lymphocytes and are surrounded by multiple layers of macrophages (PALS collapse) and there is an absence of primary follicles. Moderate numbers of neutrophils and a mild increase in macrophages throughout the red pulp vascular spaces.

 

Thymus: All thymic lobules are diminutive and are surrounded and separated by abundant adipose tissue. There is diffuse lymphoid hypoplasia with lack of a discernible cortex and medulla, and reticuloepithelial cell network is very prominent. Hassall’s corpuscles are admixed with moderate numbers of macrophages, eosinophils, and few lymphocytes.

 

Pancreas: Multifocally, moderate fibrous connective tissue surrounds pancreatic ducts and separates, surrounds, and replaces acinar cells (fibrosis). Multifocally, pancreatic acini are decreased in size (atrophy). Often, ductular epithelial cells display one of the following changes: swollen with vacuolated cytoplasm (degeneration), hypereosinophilic cytoplasm with karyorrhexis or nuclear pyknosis (necrosis), or are piled up to four layers with vesiculate nuclei (hyperplasia). Often, ductal lumina contain sloughed epithelial cells admixed with necrotic cellular debris. Occasionally, pancreatic ductal epithelial cells, including sloughed epithelial cells, contain a round to oval, 15 to 20 µm diameter, smudgy, basophilic intranuclear viral inclusion body that marginates the chromatin. Multifocally, interlobular septa are expanded by clear space with few ectatic lymphatics (edema).

 

MORPHOLOGIC DIAGNOSES: 1. Spleen, white pulp: Lymphoid hypoplasia, diffuse, severe, Arabian, equine.

2.  Thymus: Lymphoid hypoplasia, diffuse, severe.

3.  Pancreas, pancreatic ductal epithelium: Degeneration, necrosis, and hyperplasia, multifocal, moderate, with multifocal moderate fibrosis, multifocal acinar atrophy, edema, and basophilic intranuclear viral inclusion bodies.

 

ETIOLOGIC DIAGNOSIS: 1. Hereditary lymphoid hypoplasia

2. Adenoviral pancreatitis

 

CAUSE: 1. Homozygous defective DNA-PKcs

2. Equine adenovirus

 

CONDITION: Combined immunodeficiency (CID), severe CID (SCID) 

 

GENERAL DISCUSSION: 

• SCID is not a specific condition, but a constellation of entities which result in deficiencies in both humoral and cell-mediated immunity (CMI) due to failed production of functional lymphocytes
• Neutrophils, monocytes, macrophages and natural killer (NK) cells function normally; complement levels are normal
• SCID affects horses, humans, mice, and dogs, and can have an autosomal recessive, X-linked, or sporadic pattern of inheritance
• In horses:  
  1. SCID is the most important equine immunodeficiency with 0.2 to 2.3% prevalence of disease in homozygous foals and a 8 to 26% prevalence in heterozygote Arabian horses and Arabian crossbreeds; inherited as an autosomal recessive trait
  2. Foals die by five months of age as a result of infection by a variety of pathogens such as equine adenovirus, Pneumocystis carinii, Cryptosporidium parvum and Rhodococcus equi

 

PATHOGENESIS:

• SCID represents a heterogeneous group of entities, each with a different molecular basis; the common end result is absence of functional B and T lymphocytes, resulting in impairment of both CMI and humoral immunity
• The most severe form (rare) is a defect in prolymphocytic stem cells that affects both bone-marrow-derived and thymic-dependent lymphocytes
• More commonly, defects affect just T, or both B and T lymphocytes, both of which result in impairment of both CMI and humoral immunity:
  1. Defect in T lymphocytes à loss of T lymphocyte-dependent signaling to activate B lymphocytes à secondary impairment of humoral immunity
• Two major molecular mechanisms of importance in animals:
  1. Autosomal recessive defect causing inhibition of DNA-dependent protein kinase (DNA-PK), reported in Arabian foals, Jack Russell terriers, and CB-17 strain mice
  2. X-linked defect in type I cytokine receptors reported in dogs (Basset hound, Cardigan Welsh corgi)
• Synthesis of B and T cell antigen receptors requires excision and then recombination of large segments of DNA, which are recombined to form variable (V), joining (J) and diversity (D) gene segments; enzymes involved in this process include:
  1. Recombinase-activating genes 1 and 2: Make the initial DNA cuts (these enzymes are normal in SCID) 
  2. DNA-dependent protein kinase (DNA-PK): Reconnects the V, J, and D gene segments
     • In Arabian (and Arabian cross) foals and Jack Russell terriers, the specific defect in is in the gene (PRKDC) encoding the catalytic subunit (cs) of this enzyme (DNA-PKcs) (located on chromosome 9 in horses)
     • Inheritance of this defect is autosomal recessive
     • Impaired DNA-PK activity à inability to form functional V regions of antigen receptors in both T and B cells à lymphocytes unable to respond to antigens à patient cannot mount an adaptive CMI or humoral response à recurrent infections (especially viral or fungal infections which rely on CMI for clearance) à death
     • DNA-PK is also important in DNA repair, therefore there is an increased incidence of sarcoids in heterozygous adult horses (homozygous foals rarely live past 5 months)
     • In CB-17 strain mice, there is a different defect, which also results in decreased DNA-PK enzyme activity, resulting in SCID

 

TYPICAL CLINICAL FINDINGS:

• Maternal antibodies confer adequate humoral immunity to neonates, so affected animals at birth (following adequate colostrum intake) are clinically normal
• By 10 to 35 days of age, foals begin to develop any of the following, which fail to respond favorably to antimicrobial therapy
  1. Intractable respiratory disease (the most common clinical sign): Nasal discharge (often sufficiently profuse to impair suckling), cough, dyspnea, and intermittent fever
  2. Diarrhea, progressive weight loss, depression, rough haircoat, tachycardia
  3. Alopecia and dermatitis (often due to Dermatophilus congolensis)
  4. Swollen joints
• Clinical pathology
  1. Severe lymphopenia (<1000/µl is diagnostic)
  2. Leukocyte counts usually WNL (due to a compensatory neutrophilia)
  3. Hypogammaglobulinemia progressing to agammaglobulinemia following catabolism of passively-transferred antibodies (IgM declines most rapidly due to shortest half-life and is undetectable by 21 to 36 days of age in affected foals that received colostrum) 
  4. Absence of IgM from a pre-suckle foal supports diagnosis of SCID 
  5. Mild anemia (often masked by dehydration)
• Death (by 2-5 months in foals; 4 months in dogs, and 40-80 weeks for SCID mice in microisolators)

 

TYPICAL GROSS FINDINGS:  

• Characteristic lesion in foals: Bilateral cranioventral bronchopneumonia with hypoplasia of all lymphoid tissue (small spleen, lymph nodes, and thymus which may be grossly undetectable) and absence of lymphoid follicles
• Bone marrow is unusually reactive for patient’s age, with persistent hematopoiesis in cancellous bone and central femoral cavity
• Other changes related to secondary infections (e.g. omphalophlebitis, enteritis, hepatitis)

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• The primary SCID lesion is profound lymphoid hypoplasia of primary and secondary lymphoid tissue
  1. Spleen: Absence of lymphoid follicles, periarteriolar lymphoid sheaths, and plasma cells; lymphoid follicles lack connective tissue stroma (as opposed to atrophy); sinuses are moderately cellular (consisting of macrophages and extramedullary hematopoiesis)
  2. Thymus: Small lobules with no corticomedullary differentiation, very few lymphoid follicles, but presence of Hassall’s corpuscles, +/- cysts
  3. Lymph nodes: Absence of lymphoid follicles, plasma cells, and corticomedullary differentiation; collapsed medullary cords and expansion of subcapsular and medullary sinuses by macrophages and granulocytes; +/- cortical microabscesses
• Lung: Suppurative bronchopneumonia, often with basophilic intranuclear adenoviral inclusion bodies in bronchial and bronchiolar epithelium (also often found in pancreatic ducts and salivary glands)
  1. Adenoviral pneumonia may be complicated by secondary infection (Rhodococcus equi, Pneumocystis carinii)

 

ADDITIONAL DIAGNOSTIC TESTS:  

 

DIFFERENTIAL DIAGNOSIS:  

Other significant immunodeficiencies in horses:

• Failure of passive transfer: Failure to ingest and/or absorb Ig-rich colostrum à immunocompromise early in life (up to 10% of all foals)
• Selective IgM deficiency: Low or absent IgM; lymphocyte counts and B/T-cell proportions generally normal; most frequently reported in quarter horses and Arabians; secondary cases are often associated with neoplasia
• Agammaglobulinemia: Rare primary immunodeficiency of foals characterized by complete B cell dysfunction; only reported in males à likely X-linked inheritance 

 

COMPARATIVE PATHOLOGY: 

SCID in other species:

·  Dogs:

1. SCID (Jack Russell terrier): Similar to Arabian foals, autosomal recessive inheritance of defect in DNA-PKcs
2. XSCID (Basset hound, Cardigan Welsh corgi): X-linked inheritance of a mutation in the gene encoding the common gamma subunit of the IL-2, 4, 7, 9, 15, and 21 receptors (type I cytokine receptor family); the site of mutation in the gene is different for each breed
   • Inability to produce functional gamma chain of the IL-2 receptor (IL-2Rgamma) à T lymphocytes unable to respond to IL-2 (IL-2 is still produced, but T cells fail to respond) à B lymphocyte activation only occurs through T-cell-independent mechanisms (i.e. antigens) and can only synthesize IgM (unable to class switch to IgG or IgA without T lymphocyte signals) à hypogammaglobulinemia (normal IgM, decreased IgG and IgA), thymic dysplasia, failure of T cells to respond to mitogenic stimuli, increased susceptibility to infection à death by four months of age
   • Lymphopenia is less severe in XSCID than SCID, with a more significant decrease in circulating CD8+ cells (CD4:CD8 of ~15:1, normal is 2:1) 
   • Hassal’s corpuscles are not detectable in the thymus (unlike SCID in horses, but like XSCID in humans)

1. Autosomal recessive acrodermatitis in bull terrier dogs: Fatal with growth retardation, progressive acrodermatitis, pyoderma, diarrhea, pneumonia etc.; decreased plasma zinc; severe decrease in T cells in all lymphoid organs
2. Growth hormone deficiency in Weimaraners associated with low T cell counts and a small thymus

 

• Mice:  
  1. BALB/c type CB-17:  Autosomal recessive inheritance; defect in the gene that encodes DNA-PK à no functional T or B cells produced 
  2. Unique in that if maintained in pathogen-free microisolators, aging (>6 months old) CB-17 mice will begin producing some immunoglobulins and low numbers of mature T lymphocytes ("leaky SCID"), and up to 15% develop thymic lymphoma 
  3. NOD.Cg- Prkdc^scid Il2rg^tm1Wjl/SzJ (NSG, NOD scid gamma) mice have become a model of choice in several areas of biomedical research because of its ability to support engraftment of a wide range of human cell types; in this mouse, SCID refers to a defective somatic recombination at the level of T cell receptor and immunoglobulin chain loci due to homozygosity for the Prkdc^scid mutation, with consequent defective development and maturation of T and B cell clones, leading to an absence of functional lymphocytes and hypogammaglobulinemia; these mice also have absence of C5 complement component, decreased NK cell activity, defects in antigen-presenting activity, impaired macrophage function, deficiencies in cytokine signaling, and absence of NK cell development. Lactate dehydrogenase-elevating virus (LDV) is a common contaminant of mouse biological materials, and has been recently shown to cause neurological disease in SCID mice (Yang et al., Toxicol Pathol, 2022), and is a potential confounding factor in experiments using SCID models. 

 

References:

1. Durham, AC and Boes, KM. Bone Marrow, Blood Cells, and the Lymphoid/Lymphatic system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:880, 888.
2. Santagostino SF, Arbona RJR, Nashat MA, White JR, Monette S.   Pathology of aging in NOD scid gamma female mice. Vet Pathol, 2017:54(5):855-869.
3. Tilman H, Janke LJ, et al. Morphologic and immunohistochemical characterization of spontaneous lymphoma/leukemia in NSG mice. Vet Pathol. 2020; 57(1):160-171.
4. Valli VEO, Kiupel M, Bienzle D. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6^th ed. St. Louis, MO: Elsevier; 2016: 139-141.
5. Webb JL, Lattimer KS.  Leukocytes.  In:   Lattimer, KS.  Duncan and Prasse’s Veterinary Laboratory Medicine Clinical Pathology.  5^th Ed. Ames, IA: Wiley-Blackwell; 2011:78-79.
6. Snyder, PW. Disease of Immunity. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:334-335.
7. Yang P, Freeman ZT, Dysko RC, Hoenerhoff MJ. Degenerative Myelopathy and Neuropathy in NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) Mice Caused by Lactate Dehydrogenase–Elevating Virus (LDV). Toxicol Pathol. 2022;50(3):390-396.

 

 

 

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