Activation of Cytolytic Effector Cells

Abstract

Cytolytic lymphocytes can be divided into three major subgroups (Table 1). The majority of cytotoxic lymphocytes is represented by cytotoxic T lymphocytes (CTL), being CD56⁻, CD2⁺, CD3⁺, and CD16⁻. On the other hand, there are 12%–15% peripheral blood natural killer (NK) cells CD56⁺, CD2⁺, CD3⁻, and CD16⁺. The latter are characterized by their non-major histocompatibility complex (MHC) restricted killing prior to any immunization. In contrast, specific CTLs use their CD3-associated T-cell receptor structure in a MHC restricted fashion. In the middle between these two populations there is a third subset of cytotoxic lymphocytes expressing the surface antigens CD56, CD2, CD3, and CD16 [1]. These express a T-cell receptor structure and at the same time kill in a non-MHC restricted fashion and therefore are described as non-MHC restricted T lymphocytes [2]. Based on a large number of studies, the function of immune surveillance against tumors must be attributed to NK cells [3]. There is only very preliminary evidence for a specific T-cell response against tumor cells. The so-called lymphokine activated killer (LAK) cells are generated during a 5-day in vitro culture using interleukin-2 (IL-2) from the NKH1 (CD56⁺) subset of lymphocytes [4]. It is now generally accepted that LAK cells described a function only, exerted by NK cells and some non-MHC restricted T lymphocytes [5].