Functional Characterization of CD4⁺ T-Cells in Aplastic Anemia (AA)

Abstract 1340

We have examined the role of CD4⁺ T-cells in the pathogenesis of AA in 63 patients, 48 of whom were analyzed at diagnosis and 15 following immunosuppressive therapy (IST). Absolute numbers of CD4⁺ regulatory T cells (Tregs, defined as CD3⁺CD4⁺CD25^highCD27⁺Foxp3⁺) were lower in pre-treatment AA patients compared to 10 healthy donors (HDs) (5.5 × 10⁶ v 1.4 × 10⁷)(p=0.01). In patients with severe (SAA) and very severe AA (VSAA), the absolute number and frequency of Tregs were lower than non-severe AA (NSAA) (4.4 × 10⁶/L v 1 × 10⁷/L)(p=0.01) and HDs (4.4 × 10⁶/L v 3 × 10⁷/L) (p<0.001). Absolute numbers of Th1 and Th2 cells in all pre-treatment patients were higher compared to HDs (6.4 × 10⁷/L v 1.8 × 10⁷/L)(p=0.03) for Th1 and (2.6 × 10⁷/L v 2.4 × 10⁶/L)(p=0.006) Th2 cells. Although mean percentages of AA Th17 cells were higher than in HDs (1.5% v 0.15%)(p=0.04), differences in absolute numbers were not significant. Absolute numbers of Th2 and Th17 cells were increased in SAA (1.3 × 10⁷/L v 7.4 × 10⁶/L for Th2)(p=0.01) compared to NSAA (5.7 × 10⁶/L v 2.15 × 10⁶/L for Th17)(p=0.02). Ratios of Th1/Tregs (p=0.003), Th2/Tregs (p=0.02), and Th17/Tregs (p=0.001) were higher in SAA and VSAA compared to NSAA. Percentage of both activated (CD4⁺CD45RA⁻CD25^highFoxp3^high) and resting (CD4⁺CD45RA⁺ CD25^highFoxp3^low) Tregs was decreased in AA patients, compared to HDs (p=0.004 and p=0.01), whereas cytokine secreting Tregs (CD4⁺CD45RA⁻CD25^high Foxp3^low) were increased in AA (p<0.003). Sorted Tregs from AA patients did not suppress cytokine secretion by autologous or HD T effectors (Te) cells in 1:1 co-cultures, whereas IL-2 and IFN-γ secretion by AA Te (CD4⁺CD25^lowCD127^high) was suppressible by allogeneic Tregs from HDs, confirming Tregs dysfunction. AA Tregs did not inhibit either CD154 or CD69 expression on Te cells. Tregs from AA patients secreted significantly more IFN-γ, TNF-α and IL-17 (p=0.02, p=0.02 and p=0.01, respectively) after 4 hours stimulation with PMA/Ionomycine compared to HDs. Expression levels of FoxP3, ROR_□c and T-bet in AA Tregs was normal. IFN-γ secreting cells (Th1) were enriched using enrichment kit then further enriched by FACS sorting. CDR3 region products of TCR Vβ-chain were amplified using Vβ specific forward and Cβ reverse primers. CDR3 PCR products from AA patients and HDs were subjected 454 sequencing (Roche GS FLX titanium). Sequencing was performed to yield an average ‘depth’ in excess of 1000 clonally reads (1000x) for each sample specific CDR3 PCR amp icon. Reads were processed using Roche Amp icon Variant Analyzer software (AVA). Diversity of TCR receptors (measured by spectratyping and confirmed by high throughput deep sequencing) in AA Th1 cells was lower than HDs (p=0.037), as shown by the percentage and number of consensus clusters in total sequence reads. Interestingly, percentages of the most dominant CDR3 clones, revealed by high throughput sequencing, were higher in AA compared to HDs, regardless of spectratyping pattern. Global gene expression of Tregs was compared in 3 pre-IST AA patients and 5 HDs. A unique gene signature consisting of 86 genes that were significant was identified. There were 8 down regulated genes (fold change) in the pre-treatment group; PIN4 (−4.1), OR2T12 (−3.3), AMAC1 (−2.73), PERP (−2.69), UTS2 (−2.27), RNF139 (−2.13), COMMD9 (−2.09) and LOC100128356 (−2.01). The top 10 of 78 up-regulated genes in the pre-treatment group were HBB (19.5), PSME2 (13.8), CSDA (13.07), FAM127A (7.78), EXOSC1 (7.73), BPGM (7.43), CYSLTR1 (7.17), CHPT1 (6.96) and PLAC8 (6.71). qPCR analysis for CSDA, HBB, PSMiE2, PERP, PIN4, and UTS2 confirmed a similar trend to the microarray results. Interestingly absolute number of Tregs, and Th2/Treg ratio were higher in 10 IST responsive patients compared to 5 non-responsive patients (p=0.005 and 0.02, respectively).

We show that expansion of Th1, Th2, Th17, and decreased/skewed Tregs immunophenotype and function are a consistent and defining feature of SAA and VSAA. Clonal expansion of Th1 cells is likely to be antigen driven and the presence of dysfunctional Tregs aggravates this autoimmune response.

Increases of Tregs, and Th2/Treg ratios following IST predicts a favourable response to this treatment.