Gammopathy-related crystal-storing histiocytosis, pseudo- and pseudo-pseudo-Gaucher cells. Critical commentary and mini-review.

Commenting on a case report of dermal immunocytoma with crystal-storing histiocytes, a short review is given on the phenomena related to crystallization of monoclonal immunoglobulins within plasma cells, in the extracellular space, or within the lysosomal compartment of macrophages. Paraprotein crystallization is supported by hydrophobicity, poor solubility in the cold or at acid pH, and there are few reports on structural defects of crystal-forming myeloma proteins which are supposed to promote either their crystallization or impaired intralysosomal degradation. In the hitherto known cases of crystal histiocytosis, immunoglobulins of light chain type kappa have been exclusively involved in the process of macrophage storage. Accumulation of paraprotein-related crystals in macrophages may mimic the appearance of Gaucher cells or of the so-called pseudo-Gaucher cells seen in chronic myelogeneous leukaemia. With regard to their lightmicroscopical and ultrastructural differences to both, Gaucher cells and pseudo-Gaucher cells, paraproteinaemia-related crystal-storing macrophages may be denoted as pseudo-pseudo-Gaucher cells (PPGC). Human PPGC are similar to constitutive crystal-storing histiocytes known from inbred C57 BL-6 mice. The distribution of PPGC may be limited to the realm of a plasmacytoma or immunocytoma, but there are also cases with systemic involvement of the RES similar to Gaucher disease.