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Eosinophil-Associated Diseases with Dermatologic Manifestations


Key Points

  • Peripheral blood eosinophilia provides clues to diagnosis, but it is not a diagnostic marker except when levels of peripheral blood eosinophils are in the “hypereosinophilic” range as found in the hypereosinophilic syndromes.

  • Many diseases with increased peripheral blood eosinophils have accompanying tissue eosinophil infiltration, including skin, often with degranulation and loss of morphological identity of infiltrating eosinophils.

  • Eosinophils are observed in biopsy specimens of various skin lesions, with and without accompanying peripheral blood eosinophilia, and clinicopathological correlation is needed to arrive at the correct diagnosis; eosinophil-associated dermatoses include drug eruptions, arthropod bite reactions, parasite infestations (“drugs and bugs”), certain autoimmune blistering diseases, Wells syndrome, eosinophilic granulomatosis with polyangiitis (Churg–Strauss syndrome), and IgG4-related diseases.

  • Eosinophils commonly disrupt and lose their morphological integrity as they deposit toxic granule proteins and other inflammatory mediators in tissues, prominently in urticarial, eczematous, and pruritic skin lesions; therefore, the presence or absence of intact eosinophils in tissue specimens may not accurately reflect their pathogenic role in disease.

  • In patients with persistent peripheral blood eosinophilia from any cause, including the hypereosinophilic syndromes, tissue infiltration, and eosinophil-derived effector molecules may cause clinically relevant pathology, including irreversible organ damage.

Eosinophils are leukocytes with cytoplasmic granules that are named for their characteristic staining with the acidic dye, eosin. They circulate in blood as mature cells. Normally, they are not found in human tissues other than blood except in the bone marrow where they develop, in the gastrointestinal tract distal to the esophagus where they likely are eliminated, and in lymphoid tissues where they function in both innate and acquired immunity. Peripheral blood eosinophils are increased in various inflammatory diseases, classically in parasitic infections, allergic diseases and drug reactions, in hematological malignancies, and in some solid tumors. Eosinophils infiltrate tissues in response to certain inflammatory signals, which may or may not be accompanied by peripheral blood eosinophilia. Eosinophils commonly disrupt and lose their morphological identity as they deposit toxic granule proteins, and the presence or absence of intact eosinophils may not accurately reflect a pathogenic role for eosinophils in affected tissues. New therapies that target eosinophils show promise in treating eosinophil-associated diseases.

Classification

Peripheral blood eosinophilia can be transient, episodic, or persistent and can fluctuate between eosinophilia (0.5-1.5 × 10 9 /L) and hypereosinophilia (greater than 1.5 × 10 9 /L). Tissue hypereosinophilia can occur in the absence of increased blood eosinophils, although, often, at least episodic eosinophilia is present. Blood hypereosinophilia can be classified as follows:

  • Primary hypereosinophilia (clonal/neoplastic) in which eosinophils are neoplastic cells with underlying stem cell, myeloid, or eosinophil neoplasm, as classified by World Health Organization criteria

  • Secondary hypereosinophilia (reactive) in which eosinophilia is cytokine-driven in most cases by an underlying condition/disease and eosinophils are nonclonal

  • Hereditary hypereosinophilia (familial) in which a familial clustering of individuals with hypereosinophilia is found without hereditary immunodeficiency and no evidence of neoplastic or reactive conditions underlying the eosinophilia

  • Hypereosinophilia of undetermined significance in which hypereosinophilia develops with no evidence of underlying cause including no neoplastic or reactive conditions and no family history and with no organ damage; this category may be prodromal to primary or secondary forms.

Various patterns of tissue eosinophil involvement are recognized:

  • Eosinophil infiltration of few to many intact cells

  • Intact eosinophils with extracellular eosinophil granule protein deposition proportionate to the numbers of infiltrating eosinophils

  • Intact eosinophils with extensive extracellular eosinophil granule protein deposition disproportionately greater than the numbers of infiltrating eosinophils

  • Extensive extracellular eosinophil granule protein deposition with few or no infiltrating eosinophils (in this pattern, eosinophil involvement may not be recognized on histopathology examination).

Eosinophils likely are activated as they infiltrate tissues resulting in the deposition of their toxic granule proteins in many reaction patterns. Virtually every organ, including skin, is affected by inflammatory conditions accompanied by tissue hypereosinophilia.

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