Leukocyte Labeling with ⁵¹Chromium : I. Technic and Results in Normal Subjects

A method for labeling concentrated leukocytes in vitro with radioactive chromium is described. Intravascular survival data and organ distribution of the ⁵¹Cr-labeled leukocytes of ten hematologically normal subjects are presented and compared with the results of sequential or simultaneous granulocyte survival determinations using ³²DFP. Both granulocytes and lymphocytes are labeled by ⁵¹Cr. No evidence of diminished cell viability or significant elution or reutilization of the label could be demonstrated by in vitro studies. Curves of both ⁵¹Cr and ³²DFP leukocyte radioactivity in the circulation were exponential, with average T 1/2 values of 10.5 and 10.3 hours respectively, reflecting the random departure of labeled granulocytes from the circulation. An early rapid phase of label disappearance was noted in the ⁵¹Cr studies, attributable to label elution or the disappearance of labeled non-granulocytic leukocytes. Body counting rate curves during the first 12 hours parallel the disappearance of leukocyte and plasma radioactivity; a later plateau reflects radioactivity in RBC contaminating the concentrated leukocyte suspensions. No evidence of pulmonary or splenic sequestration was noted. Estimations of granulocytic pool sizes from the ⁵¹Cr data compared poorly with data derived using ³²DFP, probably as a result of the early elution of a portion of the radiochromate from the labeled cells or from the more rapid disappearance of ⁵¹Cr-labeled mononuclear cells. Epinephrine infusion after injection of labeled leukocytes produced peripheral leukocytosis, increased counting rates over the lungs, liver and heart and a marked diminution in counting rates over the spleen suggesting that a significant portion of the marginal granulocytic pool is located in the spleen.

We conclude that the ⁵¹Cr labeling technic will be useful in the study of the intravascular survival and tissue distribution of cells not labeled with ³²DFP, and that its use will supplement the study of granulocytes labeled with ³²DFP.