Abstract Body (Do not enter title and authors here): Background. While staged surgical palliation for infants born with hypoplastic left heart syndrome (HLHS) has enabled many patients to survive a previously fatal condition, there is now a growing population of HLHS children and young adults who experience significant co-morbidities and mortality secondary to their palliated physiology. Abnormalities of the immune system, including T-cell lymphopenia, have been documented in HLHS patients. However, the impact of altered T-cell homeostasis remains ill-defined.

Hypothesis. Both intrinsic and acquired alterations in T-cell sub-populations results in T-cell activation/exhaustion and a pro-inflammatory phenotype.

Methods. Peripheral T-cell sub-populations and activation/inhibition markers were quantified in HLHS (n=32, median age 1.65 years, range 0-13.2 years) and healthy, pediatric controls (n=14, median age 4.55 years, range 0.2-15.1 years) by mass cytometry (Helios Cytometry Time Of Flight, CyTOF). Comparison between HLHS and controls was performed by unpaired t-test or Mann-Whitney, based on normality.

Results. In HLHS subjects, circulating CD4⁺ and CD8⁺ populations are significantly decreased (p<0.001) compared to control. The proportions of CD4⁺HLA-DR⁺ (p<0.05) and CD8⁺EOMES⁺ (p<0.01) T cells are increased in HLHS. HLHS CD8⁺TC1⁺ populations are increased (p<0.05) while HLHS CD8⁺TC2⁺ (p<0.05) populations are decreased compared to controls. Notably, there is an increase in CD4⁺ regulatory T cells in HLHS subjects (p<0.001).

Conclusions. Immunoprofiling of peripheral T-cell populations suggests that abnormalities extend beyond a decrease in T-cell numbers: HLHS T cells demonstrate an activated and exhausted phenotype, which may impact development of co-morbidities in this population. Additional investigations are required to delineate the functional consequences of this unique HLHS immunophenotype.