Transcobalamin II: a marker for macrophage/histiocyte proliferation.

Increased blood levels of (apo-)transcobalamin II have been observed in several clinical conditions, but persistent inability to find a common denominator for this plasma protein aberration has hampered its introduction as a clinically useful laboratory parameter. Because several observations suggested a relationship between reticuloendothelial cell activity and transcobalamin II, the finding of an extreme transcobalamin II elevation in a patient with malignant histiocytosis was taken seriously. Of 14 consecutive patients with proliferative histiocytosis (8 malignant, 6 reactive), all revealed marked to extreme elevations of transcobalamin II. Macrophage/histiocyte origin of this protein is supported by a close parallelism to increased serum angiotensin-converting enzyme activity. Comparative pre- and postoperative measurements of transcobalamin II and angiotensin-converting enzyme in four patients with histiocytic proliferation who underwent splenectomy, an intervention that led to immediate reduction of the macrophage/histiocyte cell pool, revealed a parallel and impressive drop of both parameters, further corroborating the histiocytic origin of transcobalamin II. It is suggested that transcobalamin II determination provides useful information on activity and size of the macrophage/histiocyte system, and supplements measurements of the traditional acute phase reactants (e.g., C-reactive protein, red blood cell sedimentation rate).