Page 200 - AIDSBK23C

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mycobacteria in the macrophages. These macrophages are CD68 and S100 positive by
immunohistochemistry.[426]
Mycobacterium tuberculosis (MTB) is being seen with increasing frequency as a
complication of HIV infection. By computed tomographic (CT) scan, mesenteric
lymphadenopathy with low attenuation suggestive of necrosis, and occasional soft tissue
attenuation, can be due to MTB. Tuberculous peritonitis on CT scan reveals high-attenuation
ascites along with peritoneal and omental nodules.[416] The sites for involvement with
tuberculous lymphadenitis include cervical lymph nodes in virtually all cases, followed by
axillary involvement in 82%, ilioinguinal in 54%, and epitrochlear in 36%. The nodal size
ranges from 1 to 3 cm, and involvement is usually symmetrical. The presentation overlaps with
HIV lymphadenopathy.[832]
Histologically, with MTB infection there is usually a recognizable localized
granulomatous reaction, including caseous necrosis. Langhans giant cells, lymphocytes,
epithelioid macrophages, and fibrosis are present in variable numbers. Acid-fast tissue stain
reveals rod-shaped acid-fast microorganisms similar to that described in non-HIV-infected
patients. The organisms in the lesions are never as numerous with M tuberculosis as with MAC.

FUNGAL INFECTIONS.-- Lymph node involvement with the dimorphic fungi C
neoformans, H capsulatum, and C immitis is frequent because these infections are often
disseminated. The lymph nodes may be moderately enlarged and pale or mottled. Visible
granulomas are infrequent. Cryptococci appear as clusters of oval, narrow-based budding
organisms. The capsule is often missing so that the organism appears small enough to be
confused with Candida or H capsulatum.[454] Epithelioid granulomas are usually absent with
dimorphic fungi and any inflammatory reaction being sparse, giving a low power microscopic
pattern of a clear or pale zone within the node.
Fungal organisms can be best demonstrated with a methenamine silver stain. Capsular
forms of C neoformans appear prominent with PAS or mucin stains. Abundant clusters of small
intracellular organisms are characteristic of histoplasmosis. These 2 to 4 micron organisms are
usually found within macrophages. Toxoplasmosis or leishmaniasis may superficially resemble
histoplasmosis. Methenamine silver stains the cell wall of H capsulatum, while the more
delicate staining of PAS may reveal the nucleus. The microscopic appearance of large spherules
with endospores indicates C immitis.
Lymphadenopathy with Candida organisms is rare but can occur in cases with
widespread dissemination. Budding cells may be difficult to identify on hematoxylin-eosin
staining, particularly when accompanied by necrosis. Tissue sections may show pale areas of
staining in the subcapsular sinuses or sinusoids. An inflammatory reaction is usually sparse.
When pseudohyphae are not prominent, these budding yeasts can be confused with C
neoformans and H capsulatum. PAS or methenamine silver stains aid in differentiation.
Definitive distinction may require microbiologic culture.

CYTOMEGALOVIRUS.-- Cytomegalovirus is identified very infrequently in lymph
nodes, usually as part of a very widely disseminated infection. When CMV is present, it is most
often seen within endothelial cells or histiocytic clusters in subcapsular sinuses, and inclusion
bodies are not numerous. Pronounced inflammation, hemorrhage, or necrosis accompanying
CMV in lymph nodes is very uncommon.[417]