Immunofluorescent studies of urinary casts

Cylindruria

The morphological analysis of urine sediment is an essential part of urinalysis and casts are important elements of urinary sediment. Their shape is typically cylindrical, with extremities often rounded. Casts form within the renal tubules and are made of Tamm-Horsfall glycoprotein (THG). Under some physiological or pathological conditions THG fibrils aggregate giving rise to casts, whose formation is favoured by a number of factors including high urine osmolality and/or low pH. Casts can be found in normal subjects, in non-renal conditions, such as fever, dehydration, and acute heart failure as well as in virtually all renal diseases. Casts can be classified on the basis of their morphology as hyaline, granular, waxy, fatty, cellular (leukocytic, erythrocytic, epithelial), containing crystals or microorganisms, pigmented and mixed. As the correct identification of casts is crucial for an accurate and timely diagnosis of renal disorders, laboratory professionals should be trained to identify and classify casts properly.

Role of polymeric Tamm-Horsfall protein in cast formation: oligosaccharide and tubular fluid ions

BACKGROUND

In acute tubular necrosis (ATN), distal tubules are obstructed by casts formed by tubular debris, cells, and Tamm-Horsfall protein (THP). Since there are Arginine-Glycine-Aspartate (RGD) and Leucine-Aspartate-Valine (LDV) adhesive sequences in human THP, there may be direct integrin-mediated binding of tubular cells to THP. Alternatively, polymerization of THP may result in entrapment of the cells in its gel.

METHODS

Adhesion of LLC-PK(1) cells to THP-coated wells was directly measured. THP concentrate was dissolved in solutions which mimic urine from ATN (ATN-S), distal convoluted tubule (DCT-S), collecting duct (CD-S), and monomeric buffer (M buffer). THP was also denatured by either boiling or N-glycanase digestion. Gel formation of THP was then measured. Inhibition of LLC-PK(1) cell adhesion to collagen type I was measured with each solution, as well as after the collagen was pretreated with either monomeric (mTHP) or polymeric (pTHP) THP. The effect of pTHP on the settling rate of LLC-PK(1) cells in suspension was also measured.

RESULTS

LLC-PK(1) cells did not directly adhere to THP, a finding against integrin-mediated binding as a mechanism for in vivo tubular cell/THP cast formation. The high electrolyte concentration of ATN-S and CD-S, however, was associated with pTHP gel formation. Moreover, cells suspended in pTHP remained in suspension. In cell adhesion studies, mTHP attenuated cell adhesion by binding to the matrix, whereas pTHP attenuated cell adhesion by trapping cells in its gel. An active process was involved since both pTHP gel formation and attenuation of cell adhesion were abolished by boiling or oligosaccharide removal with N-glycanase digestion.

CONCLUSIONS

With renal ischemia and proximal tubule cell shedding, ATN and collecting duct fluid composition enhance THP gel formation and thus favor tubular cast formation and obstruction. The present in vitro results indicate the importance of oligosaccharide residues in mediating the effect of the THP on gel formation and potential cast formation in ATN.

Utility of immunofluorescence of urine sediment for identifying patients with renal disease due to monoclonal gammopathies

Immunofluorescence (IF) of urine sediment with antisera to light-chain immunoglobulins (LC) was used for patients with renal disease due to various monoclonal gammopathies. Urinary casts and/or masses from 20 of 27 patients (74%) of this group stained predominantly for one or another of the two LC, but from none of the 25 controls, who all had renal diseases other than monoclonal gammopathies. In all but one patient with monoclonal gammopathy the more intensely staining LC in the urine sediment was the same as that found in the paraprotein. For 68.7% of patients, the prevailing LC in urinary casts was the same as that prevailing in the casts of the renal biopsies. The sensitivity of IF of urine sediment for identifying patients with plasma cell dyscrasias was 74%, the specificity 100%, and the accuracy 86.5%. The positive predictive value was 100% and the negative predictive value was 78%. Globally, urine sediment IF was more sensitive than serum or urine electrophoresis (74% v 63% and 66.6%), but less than serum or urine immunoelectrophoresis (74% v 85%). In light-chain deposition disease, a disorder in which there are often no definite symptoms, urine sediment IF was more sensitive than both traditional methods. We conclude that IF of urine sediment is a useful test to identify patients with renal disease associated with monoclonal gammopathies.