Mast cells, eosinophils and the perfect storm of inflammation

Mast cells and eosinophils have a lot of common functions.  In allergic and inflammatory states, these cells come into physical contact with each other, as well as communicate using chemical signals called cytokines and chemokines.  Mast cells and eosinophils are often found together in affected tissues in disorders like allergic rhinitis, atopic dermatitis, and asthma.  Mast cells initiate the allergic inflammatory response once activated.  This signals for eosinophils to come to the tissue.  Increased numbers of mast cells and eosinophils are found in diseases like eosinophilic esophagitis, chronic gastritis, GI neoplasms, parasitic infections and IBD.  Both mast cells and eosinophils respond to eotaxins, molecules that draw eosinophils to the inflamed area.  So one signal causes both cell types to go to the affected tissue. 

Mast cells and eosinophils interact a lot by using chemicals.  Mast cell released heparin stabilizes eotaxins.  Mast cells produce IL-3 and IL-5, which lengthen the lives of eosinophils in tissue.  Mast cell mediator chymase suppresses eosinophil death and causes eosinophils to release several chemicals.   Tryptase can limit eosinophil activation.  In turn, eosinophils produce stem cell factor (SCF), which attract mast cells and protects them from cell death.  Both cell types express some common receptors, like Siglec-8, which induces eosinophil death and inhibits IgE-mediated mast cell activation.  Interactions between these cells increase activation and proliferation. 

Patients with SM may have another blood disorder, including CEL or hypereosinophilic syndrome (HES.)  SM-HES and SM-CEL with the D816V CKIT mutation has been found, and the mutation is present in both the mast cells and the eosinophils.  However, it is likely that the FIP1L1-PDGFRA fusion gene (an aberrant tyrosine kinase) is the cause of the coexistent eosinophilic and abnormal mast cell proliferations.  The FIP1L1-PDGFRA fusion has been found in several cell types, including neutrophils, monocytes and mast cells.  This finding is consistent with a mutational origin in a blood stem cell that makes mutated mast cells and overproduces eosinophils.  When these cells are not neoplastic, they are derived from separate stem cell lineages.

Shortly after the discovery of this fusion gene, there was significant debate over whether FIP1L1-PDGFRA+ disease was an eosinophilic neoplasm with increased mast cells or systemic mastocytosis with eosinophilia.  Patients with FIP1L1-PDGFRA+ eosinophilia have a lot of symptoms in common with SM: swollen spleen, hypercellular bone marrow, high numbers of abnormally shaped bone marrow cells, marrow fibrosis and elevated serum tryptase.  However, these bone marrows show less dense clusters of mast cells.  In some cases, mast cells were spindled and expressed CD2 or CD25.  Still, the WHO considers it a distinct entity and not a subset of SM.

In CKIT+ patients, GI symptoms, UP, thrombocytosis, serum tryptase value, and dense mast cell clusters aggregates in bone marrow are significantly increased.  Cardiac and pulmonary symptoms, eosinophilia, eosinophil to tryptase ratio, elevated serum B12 and male sex were higher in FIP1L1-PDGFRA+ group.

Eosinophilia in SM patients has no effect on prognosis.  Eosinophilia in MDS patients predicted significantly reduced survival.  In T lymphoblastic leukemia, eosinophilia was unfavorable for survival.  Density and activation of tissue eosinophils is related to disease progression in several neoplasms.  Mast cells and eosinophils are found in increased numbers in neoplastic disorders like Hodgkin lymphoma. 

Presence of FIP1L1-PGDFRA indicates treatment with imatinib (Gleevec), regardless of organ dysfunction.  It can show remission within 4 weeks, even at low doses.  Some patients with CKIT+ SM with HES or CEL have rapid and complete normalization of severe eosinophilia with midostaurin treatment. 

Reference:

Gotlib, Jason, Akin, Cem.  2012.  Mast cells and eosinophils in mastocytosis, chronic eosinophilic leukemia, and non-clonal disorders.  Semin Hematol 49:128-137.