*Purpose: Allergic disorders (AD) occur more frequently in children after transplantation (Tx) compared to healthy children, reflecting an imbalance of the immune system in the context of immunosuppression (including immune cell-depleting induction) that may also be favoured by heritable factors present before Tx. We previously found a lack of naïve regulatory T cells (Tregs) and other alterations of lymphocyte phenotypes associated with AD in children after heart Tx. We assessed whether these patterns are present pre-Tx by comparing immune profiles of children awaiting Tx with and without AD.

*Methods: This multicenter collaboration was conducted within the POSITIVE study, part of the Canadian National Transplant Research Program. Clinical data were collected into a REDCap database. Lymphocytes were isolated from pre-Tx blood via density gradient centrifugation and subtyped by flow cytometry with a focus on regulatory cells. Cell distributions were compared between patients with and without AD across kidney, heart and liver Tx groups (KTx, HTx, LTx) and in correlation to age.

*Results: Of the 114 patients, 33% experienced one or more AD pre-Tx, with prevalence of 40% in KTx-, 34% in HTx- and 21% in LTx-listed children. Of patients with one or more AD, 24% experienced asthma, 47% rhinitis and 47% eczema pre-Tx. Tregs, regulatory B cells, memory T cells and memory B cells were not significantly different between AD and non-AD groups. Within the HTx group, CD45RA+ naive Tregs were in trend less frequent in patients with AD than patients without AD (p = 0.079).

*Conclusions: AD are more common in children awaiting HTx, KTx, and LTx compared to the general population, with the lowest prevalence in LTx-listed children. The AD-associated immune phenotype alterations previously described were not present in these children, except in trend in HTx-listed children. This indicates that the AD-associated imbalance evolves or manifests with transplant and immune suppression.

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