Novel strategies to increase the growth promoting actions of the insulin-like growth factors: Immunological manipulation of IGF-I

The GH-IGF-I axis is a highly complex system, essential for normal growth and development. Previous researchers have demonstrated the potential for immunomodulation to augment the growth-promoting action of IGF-I. Passive administration of an ovine IGF-I antiserum elicits positive effects upon the growth-promoting actions of IGF-I in GH-deficient rodents (Stewart et al., 1993; Hill et al., 1998a; Pell et al., 2000) and preliminary studies indicate that active immunisation against IGF-I has the potential to promote lean growth in cattle (Hill et al., 1998a; 1998b). The current study broadly aimed to further assess the potential of immunomodulation as a means of achieving growth-promotion in cattle.

A high titre (1:16 800) bovine IGF-I antiserum was generated by actively immunising cattle against IGF-I conjugated to ovalbumin, using glutaraldehyde as the cross-linker. This antiserum exhibited a moderately high apparent affinity (2.14 x 109 L/mol) for IGF-I, negligible cross-reactivity with insulin and IGF-H, and was calculated to have a specific IgG content of 0.10%. It bound LR3IGF-I with only a 20-fold reduction in affinity, allowing affinity purification of bovine anti-IGF-I IgG using LR3IGF-I as the immobilised antigen. Epitope scanning did not reveal any dominant linear epitopes to which the bovine IGF-I antiserum bound, indicating that the epitope(s) recognised were discontinuous.

The ability of this bovine IGF-I antiserum to modulate endogenous IGF-I to promote anabolism, and the mechanism by which this might occur, were investigated in protein-sufficient and protein-restricted normal rats. In contrast to previous studies, which co-administered IGF-I and ovine IGF-I antisera to protein-sufficient dwarf rats, a distinct growth -promoting effect was not observed when rats offered isocaloric diets containing either 20% or 4% crude protein were treated with bovine anti-IGF-I IgG (7.5 μg/d) for up to 14 days. No significant change in tissue distribution of 131I-IGF-I was observed as a result of treatment. Although treatment with bovine anti-IGF-I IgG completely prevented the significant depression of voluntary food intake induced by protein restriction in an initial study, this effect was not observed in a subsequent study designed to elucidate the physiological basis of this effect.

Treatment with bovine IGF-I antiserum consistently altered the in vitro plasma binding profile of IGF-I. The presence of anti-IGF-I IgG decreased the amount of free IGF-I in the plasma, concomitantly increasing binding in the ~30-40 kDa region (presumably low molecular mass IGFBPs). During protein restriction, these effects were more marked, and associated with increased binding in the ~150-300 kDa region (presumably due to anti-IGF-I IgG binding). Degradation of 131I-IGF-I was significantly reduced by treatment with bovine IGF-I antiserum in both protein -restricted and protein -sufficient groups. However, only in the protein-restricted group was this translated into a significant effect upon total systemic clearance of 131I-IGF-I (-50%), and plasma IGF-I concentration (+60%; treatment was observed to ameliorate the depression of plasma IGF-I induced by protein restriction).

Finally, the pharmacokinetics and tissue distribution of 125I -bovine anti-IGF-I IgG were characterised over a 600 -minute period in protein-sufficient and protein-restricted normal rats. 125I-bovine anti-IGF-I IgG exhibited a far greater systemic half-life than 131I-IGF-I, but a similar initial volume of distribution. This volume was approximately 2-4 times the plasma volume and indicates that 125I-bovine anti-IGF-I IgG can readily enter the tissue compartment. Accordingly, substantial amounts of intact 125I-anti-IGF-I IgG were detected in liver, skeletal muscle, lungs, kidneys and skin 160, 260 and 600 minutes post-administration. The tissue distribution of 125I-anti-IGF-I IgG was compared to that of a non-specific IgG 160 minutes post-administration and did not differ significantly, indicating that within this time period, that the ability to specifically bind IGF-I does not affect the biodistribution of 125I-anti-IGF-I IgG.

Collectively, these results indicate that bovine anti-IGF-I IgG can sequester IGF-I in the plasma and protect it from degradation. This action results in decreased systemic clearance and an increased plasma pool of IGF-I that is readily able to enter the tissues, possibly via facilitated transfer to the low molecular mass IGFBPs. Therefore, although definition of the effective dosage is necessary, the bovine IGF-I antiserum described herein has the potential to modulate endogenous IGF-I action to promote anabolism in normal rats, particularly during nutritional restriction. Therefore, immunomodulation of IGF-I offers potential as a novel means of enhancing growth in cattle.