T Lymphopenia and Aged Immune System Characterize Immunodeficiency in Adult Patients with Telomere Biology Disorders

Immunodeficiency in telomere biology disorders (TBDs) is often observed in patients with severe phenotypes presenting at a young age, as in dyskeratosis congenita. The immune system of adult TBD patients, including risk of infection and correlation with survival has not been well characterized.

We reviewed the clinical records of 88 TBD patients (median age [range] = 35 [5-76]; 93% with a known germline mutation) followed at NHLBI since 2002. Significant infections were defined as opportunistic, recurrent (two or more severe infections in one year, three or more respiratory infections in one year, or the need for antibiotics for two months per year), or infections that required hospitalization. Longitudinal data of blood counts and T, B, and NK (TBNK) cell immunophenotyping were available for 88 and 55 patients, respectively. Lymphopenia and neutropenia were defined as ≤ 1.2 cell/mL and 0.5 cell/mL. Immunodeficiency was defined by CD8 ≤ 178 cell/µL, CD4 ≤ 334 cells/µL, or CD19 ≤ 60 cells/µL.

In this adult cohort, 32/88 (36%) had a clinically significant infection history: 8/32 (25%) opportunistic, 16/32 (50%) recurrent, and 21/32 (65%) requiring hospitalization. Of 88 patients, 41 were lymphopenic and only 3 were neutropenic. Of 55 patients with available data for TBNK subsets, 29 were immunodeficient. Twenty-three had decreased CD3 counts, including 22 patients with decreased CD4 count and 13 patients with decreased CD8 count. B lymphopenia was observed in 16 patients. Median age was similar between groups with normal and abnormal blood counts and TBNK subsets.

Lymphopenia but not neutropenia correlated with infections. Using decision tree analysis, absolute lymphocyte counts (ALC) <1.1 and <0.96 cell/µL segregated patients with worse overall survival (OS) and at increased risk of infections, respectively. CD3 lymphopenia (both CD4 and CD8) correlated with infections requiring hospitalization and opportunistic infections, both of which also associated with poorer OS. B lymphopenia only correlated with recurrent infections and did not impact OS.

Potential underlying mechanisms associated with T lymphopenia in TBDs are increased cell apoptosis due to excessive telomere shortening and an accelerated aged hematopoiesis characterized by decreased T CD4/CD8 ratio and myeloid bias of hematopoietic stem and progenitor cells (HSPC). Telomere length (TL) of T cells was similarly shortened across the entire cohort in comparison to age-matched controls, regardless of ALC and TBNK levels. TBNK immunophenotyping showed that 38/55 (69%) had a decreased CD4/CD8 ratio, a finding further validated in 6 patients by single-cell proteogenomics (scDNA) of peripheral immune subsets. In comparison to young (n=1) and older (n=1) controls, TBD patients had decreased naïve T CD4+ and CD8+ subsets, and accumulation of effector and memory cells, consistent with an aged immune system. Increased frequencies of naïve T, NK and B cells were observed in 2/6 patients, both with PPM1D or TERTp somatic mutations.

In the entire cohort, clonal hematopoiesis (CH) in MDS-related genes (particularly in U2AF1^S34 and TP53 but not PPM1D) associated with low ALC and CD3/4/8 levels (p < 0.05). scRNA-seq of HSCs from patients with germline TERT/TERC (n=2) without CH showed the lymphoid and myeloid progenitor pool (LMPP) intrinsically biased towards myeloid differentiation, with increased expression of myeloid markers in comparison to LMPP from age-matched controls. Although MDS-related mutations are known to be associated with a myeloid biased hematopoiesis, differential expression of U2AF1^S34 vs. U2AF1^wild-type LMPP by scRNA-seq were equivalent.

In conclusion, immunodeficiency in adult TBDs is characterized by T lymphopenia and likely consequent to accelerated aged hematopoiesis. Low ALC and T CD3/4/8 levels may be useful as biomarkers of increased risk of clinically significant infections and poor OS.