Serum pregnancy-associated plasma protein A (PAPPA) as a
predictor of eosinophilic Type-2 high asthma
To the Editor,
Pregnancy-associated plasma protein A (PAPPA), a metalloproteinase that
cleaves insulin-like growth factor (IGF)-binding proteins (IGFBPs) to
increase IGF availability, is expressed systemically in pregnant women
but also in other tissues (1). Higher serum PAPPA levels are reported in
patients with newly-diagnosed asthma (1) and allergic rhinitis compared
to healthy controls and are decreased following omalizumab treatment
(2). We determined whether PAPPA could represent a novel biomarker for
Type-2 (T2) asthma by exploring the relationship between asthma severity
and phenotypes of severe asthma and PAPPA gene and protein expression
(3).
We recruited 288 severe non-smoking asthma (NSA), 102 smokers and
ex-smokers with severe asthma (SSA), 86 mild/moderate non-smoking
asthmatics (MMA) and 95 healthy non-smoking controls (HC) from the
U-BIOPRED cohort (NCT01976767) (4) (Table S1 ). Transcriptomic
and proteomic profiling of blood and sputum samples and specific serum
periostin ELISA were performed (3). Gene set variation analysis (GSVA)
was used to calculate the enrichment score (ES) of 34 genes that were
upregulated following in vitro stimulation of primary human
bronchial epithelial cells with IL-13 (T2_IL-13_IVS) (3). Eosinophilic
inflammation was defined by sputum eosinophilia >1.49%
(3). Local Ethics Committees of the recruiting centres approved the
study and all participants gave written informed consent.
Sputum cell PAPPA mRNA was elevated in NSA compared to SSA, MMA and HC
subjects particularly in granulocytic asthmatics and in the
transcriptomic-associated cluster (TAC)1; an eosinophilic cluster (5)
(Figure 1A-C ). This was more pronounced with sputum PAPPA
protein analysis according to asthma severity, in eosinophilic and mixed
granulocytic asthmatics and in T2-high asthmatics identified by the
T2_IL-13_IVS signature (Figure 1D-F ).
PAPPA mRNA expression in blood cells was similar across asthma
severities, blood granulocytes and molecular phenotypes
(Supplementary Figure 1A-C ). However, serum PAPPA protein
levels supported the discrimination seen in sputum with significant
elevation seen in SA compared to HC, in eosinophilic and mixed
granulocytic asthma and in T2-high asthma (SupplementaryFigure 1D-F ).
Sputum eosinophil percentages were significantly correlated with sputum
(r=0.88, p=10-6) and serum (r=0.41,
p=10-6) PAPPA protein levels. Overall, sputum PAPPA
protein gave a greater distinction between asthma severity, granulocyte
composition and T2-high asthma than with serum although fewer samples
were available.
These results were validated in sputum from the Airways Disease
Endotyping for Personalized Therapeutics (ADEPT) study (6)
(Supplementary Figure S2 ). Elevated PAPPA protein in
the serum and sputum of severe asthmatics and in eosinophilic compared
to non- eosinophilic subjects was seen (SupplementaryFigure S2A-D ). In addition, sputum PAPPA mRNA levels were also
elevated in eosinophilic versus non-eosinophilic asthma in the ADEPT
cohort (Supplementary Figure S2E ).
The
ES score of the T2_IL-13_IVS gene signature in bronchial brushings was
significantly, but weakly, correlated with blood eosinophil counts
(r=0.329, p=10-6), serum PAPPA (r=0.356,
p=10-6), but not with serum periostin levels (r=0.07,
p-value=0.48). In contrast, the T2 IL-13 IVS ES score was strongly
correlated with sputum PAPPA levels (r=0.72, p=10-3).
Sputum PAPPA protein levels also significantly correlated with markers
of remodelling such as MMP10 (r=0.646,
p<10-6) and MET (r=0.429,
p<10-6).
Receiver-operating characteristics (ROC) curve analysis was performed
for sputum eosinophilia (Supplementary Table S2 ). The area
under the ROC curve (AUC) for serum indicated that there was no good
predictor although blood eosinophilia was the best (0.79) being
marginally better than serum PAPPA and exhaled NO (Figure 2A ).
In contrast, sputum PAPPA was an excellent predictor of sputum
eosinophilia (0.98), better than blood eosinophilia and exhaled nitric
oxide levels (Figure 2B ).
Therefore, sputum PAPPA is an excellent biomarker for sputum
eosinophilia and for T2-high asthma whilst serum PAPPA is as effective
as blood eosinophilia in predicting high sputum eosinophil levels and
with T2-high asthma.