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Crystal-storing histiocytosis is a rare disease characterized by the proliferation of non-neoplastic histiocytes that contain crystallized immunoglobulin deposits.

It is typically associated with disorders that express monoclonal immunoglobulins.

A 65-year-old man diagnosed with myelodysplastic syndrome with multilinear dysplasia was admitted for acute oligoanuric renal failure.

During admission a bone marrow study was performed due to a progressive worsening of cytopenias. The aspirate smears showed a notable increase in histiocytes with intracytoplasmic tubular images (panel A, original magnification ×50, panels B and C, original magnification ×100). No hemophagocytosis was observed. Immunophenotype study in bone marrow and serum protein electrophoresis were normal. A slight restriction for kappa light chains was detected on serum immunofixation study. The patient finally died without diagnosis.

At autopsy, intracytoplasmic crystal deposition was observed in bone marrow, spleen, liver and lymph node histiocytes. Renal intratubular crystal deposit was also identified.

These crystals were negative for light and heavy chains with immunohistochemical (panel D, original magnification x100) and immunofluorescence techniques (conventional and with pronase).

The ultrastructural study showed electron-dense crystals without internal structure (panel E, original magnification x100).

Finally, a study of mass spectrometry in renal tissue and spleen was performed, detecting the presence of constant region IgG and Kappa light chains in both tissues (panel F).

This case illustrates a case of histiocytosis by deposition of light chain crystals, associated with crystal nephropathy in the context of monoclonal gammopathy of uncertain significance.