*4.4. The Mode of HTLV-1 Infection in SS Salivary Glands*

As a potential mechanism of HTLV-1 transmission to salivary glands, a brief discussion of the concept of HTLV-1 transmission between lymphocytes is presented next. Generally, an efficient HTLV-1 infection requires cell-to-cell contact [98]; cell-free viral transmission is insufficient. HTLV-1 preferentially infects CD4<sup>+</sup> T cells but has shown a low frequency of infection of CD8<sup>+</sup> T cells. Two theories and an additional proposal have been published with regard to the cell-to-cell transmission of HTLV-1 virions. Igakura et al. proposed a concept that they named the 'viral synapse' which consists of an assembly of HTLV-1 gag protein complex and viral genomes at the microtube-organizing center, with a polarized contact surface between HTLV-1-infected and uninfected cells [103]. Another theory was proposed by Pais-Correia and colleagues, who demonstrated that extracellular matrix and linker structures including collagen, agrin, tetherin, and galectin-3 are involved in the delivery of the HTLV-1 viral particles at the contact surface [129,130]. In this system, a biofilm-like viral assembly with extracellular components on the HTLV-1-infected CD4<sup>+</sup> T cell contacts an uninfected CD4<sup>+</sup> T cell, resulting in the transmission of virions by cell-to-cell contact.

As an additional system to convey HTLV-1 virions from infected to uninfected CD4<sup>+</sup> T cells, the concept of a cellular conduit was described by Van Prooyen et al. [131]. They showed that cellular conduits that expressed HTLV-1 p8 protein with an accumulation of lymphocyte function-associated antigen (LFA-1) acted as an intermediary with respect to the transmission of HTLV-1 virions. This novel concept was introduced as a third pathway for the transmission of HTLV-1 virions in concert with the function of p8 accessory protein [132]. In coming to detection of the initial dynamic state of HTLV-1 infection of salivary glands, we investigated the involvement of these three pathways in an in vitro study as a single publication [133] by using an HTLV-1-infected cell line, HCT-5. Although agrin, tetherin, and galectin-3 were expressed on the surface of HCT-5 cells, agrin and tetherin were observed with HTLV-1 Gag-positive particles on cultured SGECs after a co-culture with HCT-5 cells

(Figure 1C). We observed no formation of virological synapses between HCT-5 cells and SGECS, although an immunofluorescence analysis suggested the possibility of the involvement of a cellular conduit (Figure 2).

**Figure 2.** A hypothetical scheme of the initial transmission of HTLV-1 virions. HTLV-1 virions exist with extracellular matrix proteins or linker proteins including galectin-3, agrin, and tetherin. After the initial contact of HCT-5 cells with SGECs, HTLV-1 virions are Table 1. virions by the extension of a long structure that is stretched from the surface of HCT-5 cells.

The handing over of HTLV-1 virions between HTLV-1-infected cells and SGECs was confirmed by electron microscopy, although we found no endogenous virions on SGECs after a co-culture with an uninfected T-cell line.
