Response to Herbert et al.

To the Editor: We thank Herbert et al.¹ for their interest in our work.² Their laboratory has shown to be instrumental in studying the effects of enzyme replacement therapy (ERT) in infants with Pompe disease. However, there are some misunderstandings about our study on adult Pompe patients and antibody formation. Below we explain these in detail.

Herbert et al.¹ suggest that our patients be reclassified based on sustained titers rather than peak titers, because “the current classification based on a single, maximal value may lead to a lack of clarity on the role of antibody titers”. We agree that the duration of high neutralizing antibody titers is important to consider. This is why we measured titers at multiple time points over a period of 3 years (Figures 1 and 3, and Supplementary Figure 1A–C in de Vries et al.²). We observed two trends: (i) a decline of peak titers over this period at a group level and (ii) relatively few patients with high (≥31,250) peak antibody titers (16 of 73; 22%). Nine (12%) of these had high sustained antibody titers. Eight patients (11%) had very high (≥156,250) peak titers, and these classified for all but one patient as sustained high. This shows that no matter how the groups are generated, in all of these cases group sizes are very small. The statistical power to analyze potential effects on clinical outcome is limited. Therefore, we have also analyzed the eight patients with a very high peak titer and seven patients with a high sustained titer on an individual basis, and we concluded that antibodies were likely to have interfered with the effect of ERT in only one patient. We previously reported on the counteracting effect of high sustained antibodies in this particular patient.³ Herbert and colleagues may have missed the fact that only a few adult patients develop high sustained antibodies, which is in contrast to the situation in classic infantile patients. A recent study by Masat et al.⁴ on behalf of the French Pompe Registry Study Group also concluded that antibodies are not a major concern in adults with Pompe disease.⁴

Herbert et al.¹ note that not all cross-reactive immunologic material–negative infantile Pompe patients developed neutralizing antibodies, while they all experienced poor clinical outcome, and that the role of neutralizing antibodies should not be overlooked. We agree that neutralizing antibodies are not the only explanation for a poor response to ERT. First, ERT does not compensate in all cases for α-glucosidase (GAA) deficiency to an activity level above the critical threshold. The reason for this could be that the dosage is too low or the therapy is inefficient due to the formation of anti-recombinant human GAA antibodies, which neutralize GAA activity and/or interfere with cellular uptake. Second, Pompe disease may have progressed too far and tissue damage has become beyond repair. Third, as yet unknown modifying factors may enhance or decrease the effect of ERT. Fourth, the lysosomal storage of glycogen in Pompe disease induces secondary cellular responses, such as a block of autophagic flux and mitochondrial dysfunction—processes bound to interfere with ERT. Evidently, antibodies are just one of several factors determining the outcome of ERT. This is also emphasized by the heterogeneous response to ERT in patients with no or low antibody titers in our study.

Herbert and colleagues¹ suggest that assay variability “appears to be associated with control reagents” rather than titering patient samples. It is unclear to us why the authors conclude this as this is misconstrued from our paper; we did use patient samples over the titer range to determine assay variability.

Herbert et al.¹ question whether the assay used in our study to measure neutralizing effects has been standardized and whether the cellular matrix could cause variability. The assay has been standardized and the same cellular matrix (fibroblasts from a classic infantile patient without any detectable GAA activity) was used in all experiments. We would like to emphasize that assessment of neutralizing effects is an important aspect to investigate the potential impact of antibodies on ERT, and we wish to promote its assessment as a standard assay whenever high antibody titers are found.

The authors also question the use of our clinical outcome measures as a readout for efficacy. We note that the outcome measures have been internationally recognized in consensus meetings and have been found suitable for the detection of changes in patient performance in response to ERT in multiple clinical studies. We recommend testing for the presence of neutralizing antibodies in the case of infusion-associated reactions and when clinical outcome declines.

Herbert et al.¹ state that “genotype alone is not responsible for immune response” and that “the observation of genotype association in LOPD should be stated with caution”. We regret what appears to be a misunderstanding of our work. We did not state in our article that genotype alone is responsible for the immune response. We did, however, state that our results should be confirmed in a larger patient group.

In summary, we have conducted an in-depth study in which we measured antibody titers and their neutralizing effects at multiple time points over a period of 3 years. This showed that titers declined on a group level, a limited number of patients developed high antibody titers, and a subset of these patients showed high sustained titers, but in only one patient was a clear impact of antibodies on the effect of ERT likely.