Objective: An important consideration in the development of T cell-based cancer immunotherapy is that effector T cells must efficiently traffic to the tumor microenvironment in order to control malignant progression. Trafficking of T cells to target tissues is orchestrated by dynamic interactions between circulating lymphocytes and endothelial cells lining blood vessels.

Methods: We examined the effect of long-duration (6 h) mild (39.5±0.5 °C) thermal therapy on lymphocyte extravasation at three distinct sites: (1) high endothelial venules (HEV) of lymph nodes (LN), that support efficient trafficking; (2) vessels in normal tissues where T cell trafficking is minimal, and (3) vessels within the tumor microenvironment that are non-permissive to lymphocyte recruitment. These studies used a combinational approach examining T cell/endothelial interactions and trafficking efficiency by intravital microscopy and short-term homing assays.

Results: These studies are based on findings by our laboratory that thermal therapy enhances the trafficking of T lymphocytes by ~2-fold across LN HEV by a mechanism dependent on upregulation of the vascular adhesion molecule, ICAM-1. Thermal therapy did not alter the composition of cells that traffic to LN, suggesting that homeostatic trafficking mechanisms were maintained. Thus, naïve (L-selectin^hiCD44^lo) and central memory (L-selectin^hiCD44^hi) cells constituted the major T cell subsets that homed across HEV under normotheric and hyperthermic conditions, while effector-memory (L-selectin^loCD44^hi) T cells were excluded. Normal vessels in extralymphoid organs did not respond to thermal therapy with respect to ICAM-1 upregulation or increased T cell trafficking. However, the intravascular display of ICAM-1 was strongly increased on vessels of colon 26 tumors (CT26) implanted at subcutaneous sites in BALB/c mice. This increase in ICAM-1 correlated with an ~5-fold increase in recruitment of T cells with an effector phenotype (i.e., L-selectin^lo, CCR7^lo, CD44^hi, CXCR3⁺, α4β7 integrin^hi, LFA-1^hi) into tumor tissues. Intravital microscopy showed that thermal therapy caused an ~5-fold increase in the frequency of T cells that exhibit ICAM-1-dependent firm sticking in CT26 tumor microvessels.

Conclusions: These studies provide the groundwork of the use of thermal therapy as an adjuvant to improve entry of tumor-reactive T cells into tumor microenvironments during immunotherapy.
(Supported by NIH grants CA79765, CA094045, and DOD grant W81XWH-04-1-0354)