Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 184

6.2
Immunogenicity

The effects of glycans on glycoprotein immunogenicity (ability to elicit an
immune response) are less clear [229]. Available evidence shows that the epito-
pes of glycoproteins consist solely of peptide elements,though the bulky,hydro-
philic and often highly charged glycans may influence the conformation and
reactivity of glycoproteins. The epitopes are retained in the deglycosylated gly-
coproteins [230], although there is some evidence that the carbohydrate moiety
may influence the immunogenicity of glycoprotein [227]. In a recent study on
immunogenicity,it has been observed that nonglycosylated hCG

b elicited better

antibody response in rats than that of glycosylated hCG [231].Proteins that are

normally glycosylated can potentially have enhanced immunogenicity when
administered in aglycosyl form due to a tendency to aggregate [232]. Proteins
without appropriate carbohydrate moieties may have altered immunogenicity.
Furthermore, absence of glycosylation can unmask peptide epitopes causing an
antibody response, as seen with GM-CSF derived from yeast but not that from
CHO cells [233].

b

6.3
Metabolic Clearance and Circulatory Half-Life

Glycans play a significant role in defining the in vivo glycoprotein clearance rate
through both specific (receptor mediated) and non-specific (physicochemical)
routes, a property critical in determining the efficacy of an injected therapeutic
protein.The clearance from circulation is particularly dependent on the presen-
ce of glycans on the outer arms [234]. Glycosylation of protein is believed to
change specific biological activity, alter diffusibility and tissue distribution and
guarantees an effective transfer to target organs.Differences in glycan structure,
degree of sialylation and number of antennae play a major role in the clearance
rate of an injected glycoprotein and cause differential effects on in vivo activity
[216, 234].
Carbohydrate-specific hepatic receptor-mediated clearance mechanisms
include the asialoglycoprotein, SO 4 -GalNAc and Man receptors [235, 236]. The
asialoglycoprotein receptor on liver hepatocytes is most significant and
accounts for the short circulatory half-life of proteins lacking terminal SA [237].
Reliable quantitative analysis of SA will be an important aspect of quality con-
trol for glycoprotein pharmaceuticals. For glycoproteins produced in recom-
binant S.cerevisiae and insect cells which possess terminal Man or Gn moieties,
the Man receptor presumably represents a major clearance mechanism [229].
The glycoprotein hormones hLH, hTSH and the uncombined -subunit syn-

thesized in pituitary have unusual Asn-glycans with terminal SO 4 -GalNAc
residues whereas the terminal residues in hFSH and in hCG synthesized in
placenta are SA and Gal. A specific GalNAcT in combination with a sulfotrans-
ferase accounts for synthesis of Asn-glycans terminating with GalNAc-4-SO 4

a