Eosinophils of the marmoset monkey (callithrix jacchus)

RATIONALE The aggregation of eosinophils in the lung is one of the characteristic hallmarks of human allergic asthma that needs to be reflected in any translational animal model. Most recently the common marmoset ( Callithrix jacchus), a small-sized New World monkey, has been introduced to model human airway diseases. As a prerequisite for developing such a model, further characterization of the marmoset eosinophils is required, especially because of the apparent difficulty to distinguish marmoset eosinophils from neutrophils with the help of conventional methods (for example hematoxylin-eosin stain (H&E)) that are used in other mammalian species. METHODS For characterization of an allergen-dependent asthma-like phenotype in marmosets, we performed bronchoalveolar lavage (BAL) and analyzed blood samples. BAL was performed in anesthetized marmosets under bronchoscopic control, and cytospots were prepared from BAL fluid. For blood sample analysis, EDTA blood was taken from the femoral veins, and blood smears were prepared. Both cytospots and blood smears were stained with standard cytological staining methods used to discriminate neutrophils and eosinophils. These staining methods included H&E, modified H&E, modified Romanowsky, May-Grünwald, Pappenheim´s, Luna, PAS reaction, Congo red, sirius red, alcian blue, and naphthol-AS-D-chloroacetate esterase as well as immunohistochemistry (IHC) with a monoclonal antibody against major basic protein. RESULTS Most classical cytological stainings did not reveal obvious differences between neutrophils and eosinophils, whereas Congo red was able to depict eosinophilic granules. Pappenheim´s stain is capable to discriminate granulocytes especially by cytoplasmic staining and naphthol-AS-D-chloroacetate esterase specifically stained neutrophilic granules, enabling a clear discrimination between marmoset eosinophils and neutrophils. IHC revealed intracytoplasmic granular staining specific for eosinophils. CONCLUSIONS In order to diagnose an asthmatic phenotype in marmoset monkeys, it is important to have reliable tools for the differentiation of the inflammatory cell population that invades lung. While a thorough differentiation of eosinophils and neutrophils is not possible using standard techniques such as H&E staining, some more reliable stains were identified to differentiate eosinophils from neutrophils. Further analysis is on-going to fully characterize marmoset eosinophils in terms of histological staining and electron microscopy of lung tissue. This will enable a direct comparison of human and non-human primate asthma to obtain a reliable asthma model