Differential Impact of Total Body Irradiation- Versus Chemotherapy-Based Myeloablative Conditioning on Supragingival Plaque Microbiota and Oral Mucositis after Allogeneic Transplantation

Oral mucositis (OM) is a common early complication of allogeneic hematopoietic cell transplantation (alloHCT), causing pain, infections, swallowing/speech impairment, prolonged hospitalization, and poor quality of life. As there are no approved prophylactic or therapeutic approaches for alloHCT-associated OM, management remains supportive and limited largely to narcotic analgesics, ice chips, and total parenteral nutrition. Some, but not all studies have found higher rates of OM after total body irradiation (TBI)- vs. chemotherapy-based myeloablative conditioning. However, methods used for mucositis assessment have been inconsistent and somewhat limited, likely leading to inconsistent results. We hypothesized that comprehensive, longitudinal mucositis assessment performed by calibrated Oral Medicine specialists will address this knowledge gap. In addition, we examined whether a signature of differential effects by the specific conditioning modality is present in supragingival plaque microbiota, a site not previously profiled in alloHCT recipients.

We conducted a prospective single-center study including longitudinal mucositis assessment using the Oral Mucositis Assessment Scale (OMAS) and supragingival plaque collection by 4 calibrated Oral Medicine specialists. We classified patients into 2 cohorts: those receiving high-dose TBI (12-13.2 Gy)-based vs. chemotherapy-based myeloablative conditioning. Mucositis assessment was performed at baseline and days +7, +14, +21, +28, and +84. Total mucositis score (range 0-45) at each timepoint was calculated as the sum of all scores (ulceration and erythema) across all sites. Supragingival plaque samples (baseline and days +14, +28, and +84) were profiled using shotgun metagenomic sequencing, targeting a sequencing depth of 20M read pairs per sample. MetaPhlAn4 was used for species-level taxonomic assignment. When the overall microbiota composition in principal coordinates analysis was significantly different between the 2 cohorts at a given timepoint, MaAsLin2 (Microbiome Multivariable Association with Linear Models) was used to identify taxonomic differences. A significance threshold of 0.05 or 0.10 was considered for P values or Benjamini-Hochberg q values (in multiple testing), respectively.

A total of 43 patients (23 females) were included: TBI cohort (N = 20; 46%) vs. chemotherapy cohort (N = 23; 54%). Most common TBI- and chemotherapy-based regimens were Cy/TBI and Flu/Bu4 in 16 (80%) and 12 (52%) patients, respectively. The chemotherapy cohort was older (median age 52 vs. 37). Acute leukemia was the most common underlying disease (78% vs. 80% of patients in the chemotherapy vs. TBI groups, respectively). Methotrexate was used as a component of GVHD prophylaxis in 70% and 75% of patients in chemotherapy and TBI cohorts, respectively. OM severity peaked at day +7 in the TBI cohort vs. day +14 in the chemotherapy cohort. Afterwards, OM improved in both cohorts and remained similar until day +84. Day +14 OM was more severe in the chemotherapy cohort compared to the TBI cohort (P = 0.004). The comparison at other timepoints was not significant. Other clinical predictors of worse OM were male sex for day +7 (P = 0.008) and older age for day +21 (P = 0.03) mucositis. There was no difference in supragingival microbiota diversity between the 2 cohorts at any of the timepoints. While baseline microbiota composition was similar between the 2 cohorts, a noticeable difference was present at days +14 (P = 0.04), +28 (P= 0.005), and even +84 (P = 0.07). Differential abundance analysis revealed enrichment of Prevotella melaninogenica and Scardovia wiggsiae in the TBI cohort and a Rothia species in the chemotherapy cohort.

In a collaborative multi-aspect longitudinal study between alloHCT and Oral Medicine teams, we demonstrated different dynamics and severity of OM after myeloablative alloHCT depending on whether conditioning was TBI- vs. chemotherapy-based. In addition, in this first profiling of the supragingival plaque microbiota in alloHCT recipients, we characterized a lasting signature of TBI- vs. chemotherapy-based conditioning on the oral microbiota. These conditioning-specific changes in the oral microbiota may have long-term implications for transplant survivors. Novel findings from this work improve current understanding of short- and long-term oral toxicities of myeloablative conditioning.

Rashidi:Seres Therapeutics: Consultancy; Emmes DSMB: Membership on an entity's Board of Directors or advisory committees.