JPC SYSTEMIC PATHOLOGY

HEMOLYMPHATIC SYSTEM

February 2024

H-N02

 

Signalment: 2-year-old male castrated domestic longhair cat

 

HISTORY: This cat was feline leukemia virus positive.

 

HISTOPATHOLOGIC DESCRIPTION: Lymph node: Diffusely effacing the cortex and medulla is an unencapsulated, infiltrative, densely cellular neoplasm composed of diffuse sheets of large lymphocytes on a pre-existing fibrovascular stroma. Neoplastic cells have distinct cell borders and scant eosinophilic granular cytoplasm. Nuclei are large and irregularly round with coarsely stippled chromatin and 1-2 prominent nucleoli.  Mitoses average 6 per individual high power field. Multifocally, neoplastic cells are within vessels and there is scattered single cell necrosis. Subcapsular and medullary sinuses contain moderate hemorrhage, small amounts of fibrin, increased numbers of histiocytes (sinus histiocytosis) that multifocally contain phagocytized erythrocytes (erythrophagocytosis) and hemosiderin (hemosiderophages). The capsule and perinodal adipose tissues are mildly expanded by variable amounts of hemorrhage, fibrin, lymphocytes and macrophages, rare neutrophils, and mild edema. 

 

MORPHOLOGIC DIAGNOSIS: Lymph node: Lymphoma, diffuse, large-cell, intermediate grade, domestic longhair, feline.

 

CAUSE: Feline leukemia virus (FeLV)

 

ETIOLOGIC DIAGNOSIS: Feline retroviral lymphoma

 

GENERAL DISCUSSION:

• Feline leukemia virus (lentivirus; genus Gammaretrovirus, family Retroviridae; enveloped RNA virus) – Causes virus-induced dysfunction, lysis, and/or neoplastic transformation of lymphoid cells leading to lymphoma, leukemia, immunosuppression with secondary or opportunistic infection, and organ dysfunction   

• Historically, up to 80% of feline lymphoma was associated with FeLV; this has been reduced due to vaccination, testing, and selective elimination of FeLV+ cats
• 50% of hemobartonellosis (Mycoplasma haemofelis) and 75% of toxoplasmosis (Toxoplasma gondii) have immune suppression from FeLV; also associated with feline infectious peritonitis, chronic oral and gingival diseases, recurrent abscesses, skin infections, etc 
• 15% of cats infected with FeLV are also infected with feline immunodeficiency virus (FIV); FIV is less infectious and less likely to cause disease, but increases risk of cat developing lymphoma  

• Main syndromes associated with FeLV include lymphoma (alimentary, thymic, cranial mediastinal, multicentric or atypical), leukemia, myeloproliferative disorders, nonregenerative anemia, panleukopenia-like syndrome, and glomerulonephritis 

• Lymphoma most common in thymus; 90% of cats with thymic or mediastinal lymphoma are FeLV positive 

• Other FeLV-associated disorders: Myelopathy, medullary osteosclerosis or myelofibrosis (associated with nonregenerative anemia and coarse hemosiderin deposition in the bone marrow), megaloblastic anemia (asynchronous maturation of erythroid precursors leading to large erythroid cells with atypical nuclei), generalized lymphadenopathy (initial lymphoid hyperplasia, followed by lymphoid atrophy later in infection), thymic cortical atrophy (occurs in all cats the die of FeLV), enteritis (diarrhea with crypt necrosis), pneumonia, giant-cell dermatosis (rare; focal to generalized hyperkeratosis, ulcers, mixed inflammation, alopecia, and keratinocyte syncytia), paw-pad “cutaneous horns,” ocular disease (Wronski et al, Vet Pathol 2023), and reproductive disorders (abortion, infertility, stillbirth)
• Morphological features used in histopathologic classification (applicable in all species and any organ) include (per PBVD, pg. 868):

• Pattern: nodular or diffuse
• Cell size: nuclei of neoplastic lymphocytes are compared to the diameter of a red blood cell and are classified as small (<1.5 x diameter RBC), intermediate (1.5-2.0x) and large (> 2 x)
• Grade: number of mitotic figures in one 400X field allows classification as indolent (0-1), low (2-5), mid (6-10) and high (>10).

 

PATHOGENESIS:  

• Four subgroups – FeLV-A, FeLV-B, FeLV-C, FeLV-T (FeLV-61C)

• FeLV-A – only transmissible type 

• Most common subgroup (100% of infected cats); causes immunosuppression and lymphoma
• Method of transmission: direct contact with virus-infected cats or from fomites containing body fluids (e.g. saliva, nasal secretions); grooming major source of infection 
• Ingested or inhaled > replicates in oral and nasopharynx mucosal epithelial cells and mucosa-associated macrophages and lymphocytes, especially in the tonsils > leukocyte trafficking to pharyngeal lymph nodes > further systemic dissemination, using primarily B-cells to traffic 
• The primarily target is bone marrow T-cells or their precursors, resulting in T-cell dysfunction and persistent infection; virus does NOT integrate into the genome to become persistent, so relies on rapidly dividing cells of bone marrow; continuous viral replication predisposes to mutations 

• FeLV-B – Arises within individual cat via recombination of (antigenic shift) of endogenous FeLV (enFeLV) genes and FeLV-A; second most common subgroup (50%); causes neoplastic transformation (especially thymic lymphoma)
• FeLV-C – Arises within individual cat via point mutation (antigenic drift) in FeLV-A env gene; uncommon (1-2%); causes anemia 
• FeLV-T – A genomic variation of FeLV-A; causes immunodeficiency syndrome 

• All subgroups use envelop glycoproteins (surface glycoprotein and transmembrane protein) to bind receptors and enter lymphocytes, macrophages, and epithelial cells; receptors include: 

• Feline thiamine transport protein (THTR1 [SLC19A2]) – used by FeLV-A
• Feline phosphate transporter proteins 1 and/or 2 (PIT1 [SLC20A1], PIT2 [SLC20A2]) – used by FeLV-B
• FeLV-C cellular receptor (FLVCR, a heme transport protein) – used by FeLV-C

• Systemic involvement coincides with immunosuppression (primarily affecting cell-mediated immunity) and lymphopenia; due to:

• Virus inducted cell lysis of T-cells > low numbers  
• Suppression of lymphokines (INF-y, IL-2) from activated T-cells 
• Production of FeLV p15e (envelop protein), which suppresses lymphocyte function (controversial) 
• Dysfunction of lymphokine-induced activation of macrophages 
• Dysfunction of neutrophil phagocytosis (persists up to a year after viremia cleared)
• Impaired NK-cell cytolytic function (but binding target cells unaffected) 

• Once cells are persistently infected, neoplastic transformation occurs (usually in the bone marrow) due to insertional mutagenesis

• Viral-produced reverse transcriptase transcribes viral RNA into proviral DNA and inserts it into the chromosomes of T-cells or other marrow cells in critical regions containing or influencing expression of oncogenes (e.g. c-myc, flvi-1, flvi-2, fit-1, pim-1, flit-1) > uncontrolled clonal proliferation without immune response induction 
• Most commonly associated with T-cell lymphoblastic lymphoma, but virus genetic variation can lead to any type of lymphoma or leukemia; insertion at flvi-1 results in B-cell lymphoma; insertion in U3 region leads to myeloid leukemia 
• Insertion in genome is random, so not all animals develop neoplasia
• Transformed cells express feline oncornavirus-associated cell membrane antigen (FOCMA) on their cell membrane; this is ONLY found on transformed cells

• Virus may be detected in megakaryocytes and platelets; thrombocytopenia may be due to direct cytopathic effects, myelophthisis or immune-mediated destruction 
• The FeLV genome contains gag, env, pol genes (as with other lentiviruses) - code for structural proteins 

• gag (core proteins) - gene codes for the viral core and nucleocapsid proteins p12, p15, p10, and p27
• env (surface proteins) - gene codes for the transmembrane protein p15e and envelope glycoprotein gp70, which are important for viral attachment and cell entry
• pol (virus replication) - gene codes for the enzymes reverse transcriptase (RNA to DNA), DNA polymerase (DNA to provirus), endonuclease (provirus into host genome), and proteases (cleavage of precursor proteins)

 

TYPICAL CLINICAL FINDINGS:

• Young cats (<3 years old)
• Weight loss
• Pleural effusion, dyspnea, regurgitation, and/or Horner syndrome due to space occupying thymic mass
• Dysmyelopoiesis with cytopenia (especially non-regenerative anemia) and/or abnormal cell morphology (e.g. macrocytes, inappropriate metarubricytosis)
• Platelet abnormalities – thrombocytopenia, thrombocytosis, giant platelets, decreased platelet function/lifespan 
• Mild to severe nonregenerative anemia (normocytic normochromic or macrocytic normochromic) +/- inappropriate rubricytosis 
• Marrow biopsy/aspiration – hypoplasia, hyperplasia or neoplasia 

 

TYPICAL GROSS FINDINGS:  

• Mucous membrane and skin pallor (anemia)
• Diffuse to tan nodular enlargement of affected organs with increased pallor +/- petechiation of liver, kidneys, spinal cord, thymus, lymph nodes, etc 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:  

 

ADDITIONAL DIAGNOSTIC TESTS:  

• IFA and ELISA tests detect the core protein p27
• Direct immunofluorescence for gp70
• PCR for FeLV-encoded DNA
• IHC 

 

DIFFERENTIAL DIAGNOSIS:

• Feline immunodeficiency virus (genus lentivirus, family retroviridae):   
  • Ingestion of blood or saliva-contaminated material, bite wounds > regional pharyngeal/tonsillar replication in T-cells or skin Langerhans cells > systemic trafficking > T-cell (CD4+) dysregulation and depletion > immunosuppression 
  • Target cells express feline CD134 (TNFRSF4) and CXCR4 cofactor; both are needed for virus attachment, binding, and entry 
  • Causes immunodeficiency syndrome, chronic stomatitis/gingivitis, wasting syndrome, neurologic manifestations, and increased incidence of lymphoma (especially if co-infected with FeLV)
  • FIV induced lymphoma (in absence of FeLV) is uncommon, tends to occur in the intestine, and is B-cell origin 
• Non-retroviral lymphoma – Most common in the intestine (usually enteropathy-associated T-cell lymphoma [EATL]; small lymphocytes resembling inflammatory bowel disease)

 

COMPARATIVE PATHOLOGY:  (Viral-induced lymphomas)

• Non-domestic felids: FeLV has been transmitted to nondomestic cats by contact or ingestion of domestic feral cats; while often asymptomatic and transient, it has caused clinical disease and mortality in free-ranging Iberian lynx, Florida panthers, and a mountain lion and has been associated with lymphoma in a single captive cheetah
• Cattle: Bovine Leukemia virus (BLV; a deltaretrovirus); causes B-cell lymphoma (enzootic bovine leukosis) in adult cattle (>2yrs; high incidence in dairy cattle)

• The viral protein, Tax, immortalizes infected cells (IgM and CD5 positive B cells)
  • Transmission typically horizontal: Blood-sucking arthropods and contaminated needles transfer infected lymphocytes in blood (rather than free virus in secretions like FeLV)
  • ~30% of cattle with BLV develop non-neoplastic persistent lymphocytosis; less than 5% of these progress to lymphoma
  • Multiple organ systems (lymph nodes, retro-orbital space, pharyngeal, abomasum, uterus, right atrium, liver, spleen, spinal cord , kidney)

• Non-human primates: Lymphomas are associated with gammaherpes infections in a variety of species 
  • Herpesvirus saimiri 2/H. ateles (Rhadinovirus): Asymptomatic in natural host (squirrel/spider monkeys respectively); lymphoma/leukemia in New World monkeys (owl monkeys, marmosets, tamarins)
  • Lymphomas increasingly reported in immunosuppressive lentiviral infections
    • Macacine herpesvirus 4, Epstein-Barr virus, Callitrichine herpesvirus 3 associated with lymphoma in immunosuppressed monkeys with SIV
• Avian: 
  • Marek’s disease (gallid herpes virus 2) - Lymphoproliferative disease of chickens which affects the peripheral nerves, gonads, skin, iris, and visceral organs
  • Avian leukosis (retrovirus)  - 16 weeks or older; 10 subgroups A-J
  • Reticuloendotheliosis - Retrovirus with unusually large avian host range
    • Mice: AKR mice develop thymic lymphoma (nearly 100%) by 6-12 months; associated with murine leukemia virus
    • Hamsters: Hamster polyoma virus causes transmissible, epizootic lymphomas in young hamsters

 

REFERENCES:  

1. Blauvelt M, Messick JB. The Lymph Nodes. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:176. 
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3. Cristo TG, Biezus G, Noronha LF, et al. Feline lymphoma and a high correlation with feline leukaemia virus infection in Brazil. J Comp Pathol. 2019; 166:20-28.

4. Cristo TG, Biezus G, Noronha LF, et al. Feline leukaemia virus associated with leukaemia in cats in Santa Catarina, Brazil. J Comp Pathol. 2019; 170:10-21.

5. Durham AC, 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; 2022:845-846.
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8. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier Limited; 2016: 627-628.

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3. Szilasi A, Dénes L, Jakab C, et al. In situ hybridization of feline leukemia virus in a primary neural B-cell lymphoma. J Vet Diagn Invest. 2020; 32(3):454-457.

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