A Look at the Lymphoid System Rose Raskin, DVM, PhD, DACVP Purdue University West Lafayette, IN

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1 A Look at the Lymphoid System Rose Raskin, DVM, PhD, DACVP Purdue University West Lafayette, IN The lymphoid organs commonly biopsied include the peripheral and internal lymph nodes, spleen, and occasionally the thymus. Lymph nodes Indications for lymph node biopsy Lymphadenomegaly or enlargement of one or multiple lymph nodes may be detected by palpation or by radiography and ultrasonography. Evaluation of metastatic disease involves evaluation of the lymph node(s) draining the primary lesion. (see Table 1) Classification of lymphoma may be enhanced by the cytologic features stained with routine stains, or by cytochemical and immunocytochemical stains to distinguish B and T cell subtypes. The latter stains are performed at specialized laboratories. Histopathology is recommended for equivocal cases to demonstrate architectural changes. Biopsy sites Popliteal and prescapular are the preferred biopsy sites for generalized lymphadenopathy. Submandibular lymph nodes are frequently enlarged and reactive due to their constant exposure to antigens making them a poor choice for biopsy in generalized lymphadenomegaly. Table 1. Selected peripheral lymph nodes in the dog Lymph Node Location Drainage Features Mandibular (submandibular) Superficial Cervical (prescapular) Axillary Superficial Inguinal Group of 2-4 nodes located ventral to the angle of the jaw Group of 2-3 nodes located in front of the supraspinatus muscle 1-2 nodes located caudal and medial to the shoulder joint 2 nodes located in the furrow between the abdominal wall and the medial thigh Includes most of the head, including the rostral oral cavity Includes the caudal part of the head (pharynx, pinna), most of the thoracic limb, and part of the thoracic wall Includes most of the thoracic wall, deep structures of the thoracic limb and neck, and the thoracic and cranial abdominal mammary glands Includes the caudal abdominal and inguinal mammary glands, ventral half of the abdominal wall, penis, prepuce, scrotal skin, tail, ventral pelvis, and medial part of the thigh and stifle Popliteal 1 node located behind the stifle Includes areas distal to the stifle Biopsy procedure considerations The size of the lymph node should also be considered. Very large nodes may yield misleading information as it frequently contains necrotic or hemorrhagic tissue. The center of a very large lymph node should be avoided during aspiration. A slightly enlarged lymph node is preferred; a sample from more than one location is desirable. For aspirate smears use a 22 gauge needle alone or together with a 6 or 12 ml syringe. Anesthesia is seldom required unless the patient is uncooperative; discomfort is limited to skin penetration. The skin over the lymph node is prepared as for a surgical procedure. The lymph node is immobilized between fingers and the needle is directed into the parenchyma in several directions. This technique of fine needle capillary sampling is especially helpful in obtaining minimally blood contaminated specimens from organs such as the spleen and liver. Alternatively, a syringe may be added to the needle or butterfly catheter to provide negative pressure. Use quick, sharp, and multiple withdraw motions of the plunger. Release the pressure on the plunger before removing the needle to avoid splattering the material within the syringe. Reattach an air-filled syringe and expel the needle contents onto the approximate center of a glass slide. The aspirate will appear creamy white, watery to viscous indicating a cellular sample. Gently squash the material with a second slide, sliding them apart horizontally. Dry smears rapidly with a hair dryer to avoid cell shrinkage. For impression smears from an excisional or incisional biopsy, it is important to blot excessive tissue fluids before smear preparations are made to increase the cellular yield. Blot the cut surface of the excised lymph node on a paper towel, and then touch it gently to a glass slide. KEY POINT: Remember to keep cytologic preparations away from formalin fumes to avoid staining problems. Cytologic and histopathologic samples must be mailed separately when submitted to a referral laboratory. 1

2 Cytodiagnostic groups for lymph node cytology 1. Normal 2. Hyperplasia or reactivity 3. Inflammation 4. Lymphoma 5. Metastatic neoplasia Cytologic interpretation of lymph nodes Normal lymph node Small, well-differentiated lymphocytes which measure times the diameter of a RBC, in the dog, should compose more than 90% of the population. The chromatin of these cells is densely clumped with no visible nucleoli. Cytoplasm is scant. These cells are the darkest staining of all the lymphocytes. Medium (2-3 times RBC) and large (>3 times RBC) lymphocytes may be present in low numbers (<5-10%). Their nuclei have a fine, diffuse and light chromatin pattern. Nucleoli may be prominent. The cytoplasm is more abundant and often basophilic. Mature plasma cells represent a small portion of the cells found. Chromatin is densely clumped and often the nucleus is eccentrically placed within the abundant deeply basophilic cytoplasm. A pale zone or "halo" is seen perinuclearly which indicates the Golgi zone. Occasional macrophages (histiocytes) can be found as large mononuclear cells with abundant light cytoplasm, often containing debris. Nuclear chromatin is finely stippled and nucleoli may be found in activated macrophages. Occasional mast cells and neutrophils also may be present. Reactive or hyperplastic lymph node Small lymphocytes still predominate, but there is an increase in medium and/or large cell types, up to 15% of the total cell population. Plasma cells are mildly to markedly increased in number and may be shifted toward immaturity. Some highly activated plasma cells are termed Mott cells characterized by abundant cytoplasm filled with multiple large spherical pale vacuoles that represent immunoglobulin secretions. Macrophages may also increase due to antigen stimulation. Neutrophils or eosinophils also may increase in number. However these cells occur in lower numbers than expected for lymphadenitis. Reactive lymph nodes are associated with local or generalized conditions. Local conditions include neoplasia, infection, immune-mediated disease, and other inflammatory situations. Generalized conditions include infectious, immunemediated, and an idiopathic enlargement in cats. In the latter condition, cats present with marked enlargements of lymph nodes that histologically resemble lymphoma. These cases generally spontaneously regress in one to 17 weeks. In one study, the majority of cats were FeLV positive and one of 14 cats progressed to lymphoma. Lymphadenitis Purulent lymphadenitis composed of >5% neutrophils may be associated with bacterial, neoplastic, or immune-mediated conditions. Eosinophilic lymphadenitis involves >3% eosinophils and is often related to allergic dermatitis, mast cell tumor, hypereosinophilic syndrome, feline fibrosing eosinophilic gastroenteritis, and certain eosinophilopoietin-producing lymphomas. Histiocytic or mixed cell lymphadenitis involves moderate to marked increases in macrophages without or with neutrophils, respectively. Conditions associated with this inflammatory response include 2

3 systemic fungal infections (e.g., blastomycosis), other fungal infections, mycobacteriosis, leishmaniasis, salmon disease, and pythiosis. When epithelioid macrophages are noted along with neutrophils, the term pyogranulomatous may be used to characterize the lymphadenitis. Lymphoma (less preferred term is lymphosarcoma) The predominant cell is usually an immature lymphocyte, since small well-differentiated lymphocytes are infrequently considered neoplastic in the dog or cat. The population is often homogenous. Medium or large sized lymphocytes account for 60-90% of the total cells. A micrometer such as an erythrocyte may be used to determine the size of the lymphocytes present. Small lymphocytes in the dog are x RBC diameter, while medium are 2x and large >3x. Mitotic figures may be frequent (2 or greater per five fields at 40x or 50x) and suggest a high turnover rate of the cells. Lymphoglandular bodies result from the rupture of lymphocytes and appear as small platelet-sized blue-staining cytoplasmic fragments within the background of the preparation. Although they may be seen in benign lymph node conditions, a higher frequency is expected in lymphoma due to the immaturity and fragility of these cells. The lyzed nuclei may appear as lacy eosinophilic material. Prognosis or response to treatment for canine lymphoma has been associated with immunophenotype, clinical substage, and prior use of corticosteroids. B-cell types generally occur in 75% of the canine lymphoma population and T-cell types in the remaining 25%. In one report, T-cell lymphomas were at significantly higher risk of relapse (52 vs 160 days) and early death (153 vs 330 days) compared with B-cell lymphoma following therapy. However, B is not always better and T is not always terrible as studies have shown. Veterinary medicine has adopted use of the current 2008 human World Health Organization classification system for lymphoid neoplasms which separates these neoplasms into distinct disease entities. This system is based on morphology, clinical presentation, immunophenotyping, genotyping, and biologic behavior of the disease. (See Table 2) Surgical removal and histologic examination of the lymph node is recommended in all questionable cases to make a definitive diagnosis. To further aid in prognosis and disease classification, immunophenotyping should be performed. When morphology or immunophenotyping cannot distinguish neoplasia or lymphoid origin, PCR techniques for B and T cell receptor clonality are considered but these tests are not without some problems. Table 2. Subtypes for canine lymphoid neoplasms by 2008 WHO classification scheme 3

4 Metastatic lymph node Metastasis is suggested by the presence of a cell population not normally expected in a lymph node e.g., epithelial cell clusters. These foreign cells often appear abnormal with several cytologic features of malignancy. The remaining lymphoid population may appear reactive with cell types as described above under reactive lymph node. The metastatic neoplasm may replace the lymph node parenchyma completely and in so doing interfere with the cytologic identification of the tissue as lymph node. Spleen Indications for splenic biopsy Splenomegaly may be detected by palpation or by radiography and ultrasonography. Abnormal imaging features suggest the presence of hyperplasia or infiltrative processes. Evaluation of hematopoiesis may be indicated with bone marrow disease. Additional considerations for spleen evaluation Aspiration may be performed in cases of thrombocytopenia, but body movements should be minimized, either by manual restraint or sedation. The needle and syringe may be coated with sterile 4% disodium EDTA prior to aspiration to reduce the clotting potential of the specimen. A inch, 22 or 23 gauge needle may be used alone or attached to a hand-held 12 ml syringe or aspiration gun. In some cases, it may be preferable to use a inch spinal needle. The animal is placed in right lateral or dorsal recumbency and the area over the site is prepared surgically. The site is carefully determined by palpation or ultrasonography. The nonaspiration method is suggested with vascular sites such as the spleen to reduce blood contamination. Cytodiagnostic groups for spleen cytology 1. Normal tissue 2. Hyperplasia or reactivity 3. Inflammation 4. Malignant neoplasia 5. Hematopoietic tissue Cytologic interpretation of the spleen Artifacts Commonly Encountered: Ultrasound gel appears as particulate eosinophilic material similar to stain precipitate. Sheets of normal mesothelium from the splenic capsular surface are seen with incisional and excisional biopsies. Normal splenic tissue Sheets of normal mesothelium from the splenic capsular surface are encountered with incisional and excisional biopsies Small lymphocytes predominate with occasional medium and large lymphocytes present. A few macrophages and plasma cells may be seen along with rare neutrophils and mast cells. Macrophages may contain small amounts of phagocytized debris, compatible with hemosiderin. Small amounts of reticular tissue with macrophages and stroma in an aggregated fashion Splenic hyperplasia or reactive spleen Small lymphocytes still predominate but there is an increase in medium and large lymphocytes. Macrophages and plasma cells are commonly observed. Associated with the macrophages may be reticular stroma appearing as basophilic fibrillar or spindle shaped elements. Hemosiderosis may be more noticeable with large amounts of coarse dark granules. Increased numbers of mast cells and neutrophils may be observed. 4

5 Hyperplasia may result from antigenic reaction to infectious agents or presence of blood parasites. Splenitis An inflammatory response is likely associated with splenic hyperplasia. Macrophages often increase in number to systemic fungal infections e.g., histoplasmosis, protozoal infections e.g., cytauxzoonosis, and leishmaniasis. Extramedullary hematopoiesis This was the most common cytologic abnormality in one study accounting for 24% of the patients. While precursors from all three cell lines may be observed, erythroid cells are the most common with metarubricytes, rubricytes, and prorubricytes present. Care must be taken as erythroid precursors and lymphoid precursors appear very similar. Conditions associated with extramedullary hematopoiesis include: chronic hemolytic anemias, myeloproliferative disorders, and lymphoproliferative disorders. Neoplasia (primary or metastatic) In myeloproliferative disorders, expect to find immature hematopoietic cell types. Malignant histiocytosis presents with bizarre and immature macrophages, often with evidence of marked erythrophagocytosis. Lymphoid neoplasia includes lymphoma and plasmacytoma (extramedullary myeloma). A large granular cell lymphoma arises primarily from the spleen to infiltrate Hemangiosarcoma the blood but not typically the bone LGL marrow. Mast cell tumor may be primary or secondary. Hemangiosarcoma is a common primary or metastatic neoplasm. Cells are large, individual, spindle to stellate with indistinct cytoplasmic borders. The cytoplasm is often vacuolated and basophilic. Other mesenchymal neoplasms that occur in the spleen include fibrosarcoma, leiomyosarcoma, and myelolipoma. Thymus Indications for thymic biopsy Enlargement of a cranial mediastinal mass may be detected by radiography and ultrasonography, often producing signs of dyspnea, pleural effusion, and dysphagia (swallowing difficulties). Abnormal imaging features suggest the presence of hyperplasia or infiltrative processes. Cytologic interpretation of the thymus Normal The cell types are similar to the lymph node with the predominance of small lymphocytes and occasional mast cells present. Variable amounts of thymic epithelium are found which may form tight balls termed Hassall s corpuscle. These appear as epithelioid macrophages with abundant pale blue cytoplasm with junctional attachment to each other. Neoplasia The homogenous appearance of lymphocytes occurs with thymic lymphoma. The cell type involved most resembles the medium size lymphocyte found in lymph nodes. These tumors have been associated with hypercalcemia. Thymoma involving epithelial cells may be of two forms, epithelial thymoma or mixed epithelial and lymphoid thymoma determined by the relative numbers of these two cell types. The epithelial cells appear as large cohesive, pale, mononuclear cells that resemble epithelioid macrophages. Large numbers of well-differentiated mast cells are often common within thymomas. Myasthenia gravis and pure red cell aplasia are paraneoplastic syndromes associated with thymoma in addition to hypercalcemia. 5