Thymus diseases

From Felipedia

Histopathology of thymus from normal (A) and irradiated feline patients (B). Note the marked reduction in thymic size, overall hypocellularity, and loss of corticomedullary distinction of the irradiated patient. Courtesy Kuhnt et al (2009)

Located in the anterior mediastinum and caudal cervical region, the thymus is the primary lymphoid organ responsible for the production of a diverse repertoire of immunocompetent and self-tolerant T lymphocytes (T-cells)^[1].

T-cell differentiation, maturation and proliferation occur as developing cells traffic through the thymic cortex and medulla in response to a multitude of cytokines and secreted factors^[2]. The thymus undergoes physiologic involution due to aging, but also undergoes pathologic atrophy due to a variety of causes, including infectious diseases, endocrine disturbances, nutritional disorders, chemotherapeutics, or radiation injury^[3].

The two leading causes of thymic disease are FIV and lymphoma:

• feline immunodeficiency virus (FIV) infection causes significant alteration and destruction of the thymus, leading to compromise and dysfunction of the host immune system and rendering individuals susceptible to opportunistic infections, primarily through a reduction in CD4+ T cells and an impairment of cell-mediated immunity, similar to individuals with AIDS. FIV-induced thymic lesions include reduction in thymic volume and weight, decreased cellularity, increased apoptosis, and decreased distinction of the corticomedullary junction^[4]. A striking finding is the spontaneous recovery in thymus weight that occurred postnatally after FIV inoculation during the second trimester of pregnancy in Queens. This difference may be due in part to the kinetics of prothymocyte supply in the fetus versus the juvenile^[5].
• Feline lymphoma occurring in the thymus is primarily observed in young male cats. This lymphoma occurs due to neoplastic transformations occurring in T-lymphocytes. Neoplastic transformation is triggered by a strain of feline leukemia virus (denoted LC-FeLV) which induces thymic lymphosarcoma in the domestic cat with short latency^[6]. Since, this type of lymphoma develops in the chest cavity; the symptoms are very much common to those observed during severe respiratory infections. The most common symptoms observed during feline thymus lymphoma include unusual swelling of the thymus gland, accumulation of fluid around the lungs, difficulty in breathing and coughing. The anterior and posterior mediastinal lymph nodes also get infected with the lymphoma resulting in a non-compressible thoracic region^[7].

References

1. ↑ Pedersen, NC et al (1987) Isolation of a T-lymphotropic virus from domestic cats with an immunodeficiency-like syndrome. Science 235:790-793
2. ↑ Lee, IT et al (2002) Prevalence of feline leukemia virus infection and serum antibodies against feline immunodeficiency virus in unowned free-roaming cats. J Am Vet Med Assoc 220:620-622
3. ↑ Pearse, G (2006) Histopathology of the thymus. Toxicol Pathol 34:515-547
4. ↑ Obert, LA, & Hoover, EA (2000) Relationship of lymphoid lesions to disease course in mucosal feline immunodeficiency virus type C infection. Vet Pathol 37:386-401
5. ↑ Johnson, CM et al (1998) Biphasic Thymus Response by Kittens Inoculated with Feline Immunodeficiency Virus During Fetal Development. Vet Pathol 35:191-201
6. ↑ Levy LS, Lobelle-Rich PA, Overbaugh J. (1993) Flvi-2, a target of retroviral insertional mutagenesis in feline thymic lymphosarcomas, encodes bmi-1. Oncogene. 8(7):1833-8
7. ↑ Medicineamigo.com