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Binding, uptake and intracellular processing of polymeric rat immunoglobulin A by cultured rat hepatocytes.

The subcellular localization of 3H-labelled polymeric IgA (pIgA) and anti-secretory component IgG (anti-SC IgG) accumulated by cultured rat hepatocytes has been studied by means of cell fractionation techniques. 
 
 
 
When postnuclear supernatants from cells incubated at 37 °C with [3H]pIgA are analysed by isopycnic centrifugation in sucrose gradients, the label distributes partly like 5′-nucleotidase, the plasma membrane marker enzyme and is found partly at lower densities around 1.13–1.14 g/ml which could correspond to galactosyltrans- ferase, the Golgi marker. This latter labelled material is associated with structures which equilibrate at these densities rather than to label released from damaged organelles since similar distributions are observed after centrifugation of fractions freed from soluble material or during flotation experiments. In addition these struc- tures do not correspond to Golgi-related vesicles since after treatment with low concentrations of digitonin, the label becomes dissociated from galactosyltransferase and is shifted to higher densities where it accompanies 5′-nucleotidase, After longer continuous incubations in the presence of [3H]pIgA or of 3H-labelled anti-SC IgG, high proportions of the cell-bound labelled material are found associated with the lysosomes. These results are in agreement with the fact that anti-SC IgG is taken up specifically by hepatocytes and digested in a process which can be inhibited by 100 pM chloroquine, a drug known to inhibit the intralysosomal proteolysis. 
 
 
 
On the basis of these results, different models can be envisaged to explain the intracellular transport of pIgA. The first model implies two distinct but parallel routes, one more rapid which escapes fusion with lysosomes, and delivers secretory IgA (sIgA) or anti-SC IgG at the biliary membrane, another which leads to a rather slow but progressive accumulation of the immunoglobulins in lysosomes wherein they are digested. In the second model, pIgA would first be collected in specialized vesicles equilibrating at rather low densities in sucrose gradients, and from which they would be transported either to lysosomes or to the bilecanaliculi as in the preceding model. Finally, the third model implies only one cellular route but two stations. These specialized vesicles, enclosing pIgA or anti-SC IgG, would first fuse with lysosomes where part of their content would be discharged; they would then dissociate from lysosomes, migrate in the cytoplasm and fuse with the biliary membrane where sIgA and immune complexes SC-anti–SC IgG would be released as a result of the action of bile.