Failure of CTL to Control Ocular Tumors Is Associated With Reduced Intratumoral F4/80+ Macrophages That Demonstrate Impaired Tumoricidal Activity

Abstract

Purpose: : To determine mechanisms which inhibit CTL effector function within the ocular tumor microenvironment.

Methods: : Ovalbumin (OVA) specific CD8+ CTL (3.0 X 10⁶ cells) were intravenously transferred into C57Bl/6 mice before or after injection of OVA-expressing tumors (E.G7-OVA) in the anterior chamber (a.c.) or skin. Flow cytometric analysis was employed to: enumerate ocular tumors, track and functionally phenotype transferred CTL, determine the cellular composition of each tumor microenvironment and evaluate the tumoricidal activity of infiltrating CD11b+ myeloid cells.

Results: : CTL transfer before injection of 10⁶ E.G7-OVA cells in the skin completely prevented tumor growth whereas the same treatment before injection of 10⁴ E.G7-OVA in the a.c. only delayed ocular tumor growth kinetics. CD8+ CTL infiltrated established skin and eye tumors and displayed similar effector function, as measured by lytic granule release and IFN-γ production. Correspondingly, established skin tumors were completely eliminated in 80% of the CTL transferred mice. In contrast, ocular tumor growth was only reduced by 3-fold despite lower tumor burden. CD11b+ Myeloid cells infiltrating skin tumors were primarily F4/80+, GR-1- cells (71.02 ± 4.1 % of CD11b) whereas F4/80-, Gr-1+ predominated in ocular tumors (76.5 ±11.4 %). In vitro tumor growth was markedly inhibited by F4/80+ cells (96% inhibition) and to a lesser extent by GR-1+ cells (44 % inhibition) isolated from skin tumors of CTL transferred but not by untransferred mice, whereas tumor growth was unaffected by ocular tumor associated F4/80+ cells or GR-1+ cells.

Conclusions: : Skin tumors are sensitive to CD8+ CTL responses whereas ocular tumors are resistant. Skin tumor associated F4/80+ macrophages are activated by CD8+ CTL to become tumoricidal effectors. Reduced numbers of F4/80+ macrophages with limited tumoricidal activity within the ocular tumor microenvironment reduce the effectiveness transferred CTL.

Keywords: tumors • immunomodulation/immunoregulation • inflammation