The In Vivo infected Clones Have Mixed Immunophenotype and Malignant Transformation Is Characterized By Dominant Growth of CD4+CCR4+CD26-CD7- Cells in Human T Lymphotropic Virus Type 1 Infection
Authors: Kagdi Huseini Hatimbhai , Taylor Graham Phillip 
Published: October 2019

Abstract
Aggressive Adult T cell leukemia/lymphoma (ATL), a human T lymphotropic virus type 1 (HTLV-1) -associated disease, has a poor prognosis. There is an urgent need for effective prevention and treatment. Aggressive ATL develops in patients with non-malignant HTLV-1 (asymptomatic carriers (AC) and patients with HTLV-1 associated myelopathy (HAM)) over decades with evolution from high proviral load (PVL) and non-dominant clonal growth through emergence of dominant clones and indolent to aggressive ATL. A single dominant clone is a major contributor to HTLV-1 infection burden in ATL despite a polyclonal background whilst hundreds of small clones contribute relatively equally to infection burden in non-malignant HTLV-1 infection(1). CD4+CCR4+CD26-CD7- is the dominant immunophenotype of lymphocytes in patients with ATL (ATL cells) but infected cells with similar immunophenotype ('ATL-like' cells) are also present in patients with non-malignant HTLV-1 infection(2). We hypothesize that 'ATL-like' infected cells harbor: larger HTLV-1 infected clones detected in total PBMC and are more oligo-clonal compared to total PBMC in patients with high PVL non-malignant HTLV-1 infection and; only the dominant clone detected in total PBMC in patients with ATL. The aim is to study the relationship between clonality and immunophenotype in patients with high PVL non-malignant HTLV-1 infection and ATL.
Clonality analysis was performed on total PBMC by ligation mediated PCR followed by high throughput sequencing (LMPCR-HTS) and on sorted CD4+CCR4+CD26-CD7- cells by LMPCR-HTS and T-cell receptor (TCR) sequencing in ACs (four and nine respectively), patients with HAM (four and nine respectively) and patients with ATL (four and nine respectively). Clonality analysis within CD4+CCR4+CD26-CD7- cells, similar to PBMC, showed hundreds of non-dominant clones (relative abundance of largest clone ≤ 25%) in 83% of patients with high PVL non-malignant HTLV-1 infection and a dominant clone (relative abundance of largest clone ≥ 25%) over a background of non-dominant clones in 17% and 100% of patients with non-malignant HTLV-1 infection and ATL respectively. CD4+CCR4+CD26-CD7- cells contributed a median of 95% of the largest PBMC clone in patients with ATL and a median of 23% of the largest PBMC clone in 63% of patients with non-malignant HTLV-1 infection. Cells which do not have 'ATL-like' immunophenotype (CD4+CCR4- and CD4+CCR4+CD26-CD7+) were also detected alongside CD4+CCR4+CD26-CD7- cells within individual clones in patients with non-malignant HTLV-1 infection as shown in figure 1. Three out of eighteen patients with high PVL non-malignant HTLV-1 infection had two or more dominant clones, one of which showed signs of overt ATL 12 months after the sample data. Clonality analysis in an patient with incident aggressive ATL showed presence of two dominant clones with two integrated provirus each 3 months prior to diagnosis and single dominant clone at presentation, remission and relapse as shown in figure 2. There was no significant difference in distribution of viral integration site by LMPCR-HTS or TCR repertoire in patients with non-malignant HTLV-1 infection and ATL.
In summary, 'ATL-like' cells are derived from relatively small non-dominant clones in the majority of patient with high PVL non-malignant HTLV-1 infection and from a dominant clone over a non-dominant background in patients with ATL. The dominant clone in patients with ATL was comprised almost universally of 'ATL-like' cells whilst the large PBMC clones in patients with non-malignant HTLV-1 infection had cells with a mixed immunophenotype including 'ATL-like' and non- 'ATL-like' cells. Oligoclonal proliferation of 'ATL-like' cells precedes monoclonal proliferation at clinical presentation of ATL. Clonality analysis in patient with high PVL non-malignant HTLV-1 infection might help in further prognostication in malignant risk.
Cook LB, Melamed A, Niederer H, Valganon M, Laydon D, Foroni L, et al. The role of HTLV-1 clonality, proviral structure, and genomic integration site in adult T-cell leukemia/lymphoma. Blood. 2014;123(25):3925-31. Kagdi H, Demontis MA, Ramos JC, Taylor GP. Switching and loss of cellular cytokine producing capacity characterize in vivo viral infection and malignant transformation in human T- lymphotropic virus type 1 infection. PLoS pathogens. 2018;14(2):e1006861.

Disclosures
No relevant conflicts of interest to declare.