Proinflammatory factor mediated lymphocyte activation-the pivotal role of leukotriene B4

Abstract

Epstein-Barr virus (EBV) is ubiquitous in the human population. More than 90% of the individuals are virus carriers. The outcome of the first encounter with the virus is highly variable. It can occur unnoticed, but if infection is delayed until adolescence it causes the infectious mononucleosis syndrome in about half of the cases. The decisive mechanism that arrests EBV induced B cell proliferation is attributed to both innate and EBV specific cellular immunity.

EBV specific immunological memory is not transferred from mother to child. Therefore, cord blood mononuclear cells (CBMC) are well suited for analysis of cellular interactions in primary infection. We found that the immunomodulators, PSK and Trx80 inhibited the EBV induced B cell proliferation. Both PSK and Trx80 activated monocytes to produce cytokines in the presence of activated T cells. PSK induced predominantly IL-15 while Trx80 induced IL-12. Both cytokines induced functional activation of the T cells. PI 3-kinase and ROS were involved in the PSK induced activation of monocytes. When the cultures containing activated T cells were restimulated with autologous EBV transformed B lymphocytes, specific cytotoxicity was generated in the cultures. By activation of innate immunological mechanisms it was possible to generate EBV-specific T cell response in CBMC cultures.

In the continuing studies, we found that NK cells were essential for cell mediated inhibition of the proliferation of EBV infected B lymphocytes, through the production of IFN-gamma. In the NK cell depleted cultures, the production of IL-15 and IL-12, T cell activation and inhibition of EBV induced B cell proliferation were reduced and cytotoxic T cells could not be generated. These functions were restored by addition of IFN-gamma to the NK cell depleted cultures.

Further we demonstrated that leukotriene B4 (LTB4) was involved in the effect of the PSK and Trx80. LTB4 was detected in the medium, and T cell activation was compromised by addition of leukotriene biosynthesis inhibitors. BLT1, the high-affinity receptor of LTB4 was expressed on T cells in the infected cultures. Moreover, we found that LTB4 added to infected cultures, which did not receive the PSK and Trx80, induced functional activation of the T cells. LTB4 activated the monocytes and acted directly on the T cells. In consequence, addition of LTB4 resulted in the inhibition of the EBV induced B lymphocytes proliferation. Specific cytotoxicity could be generated by restimulation of the T cells. The experiments showed successive stages of T cell activation in acquisition of their immunological effector function. This is orchestrated by complex cellular interactions, and autocrine loops mediated by soluble factors - here IFN-gamma, IL-15, IL-12 and LTB4.

We also studied the function of LTB4 in B-cell chronic lymphocytic leukemia (B-CLL) cells. B-CLL cells produced LTB4 and expressed BLT1. Specific leukotriene biosynthesis inhibitors counteracted CD40-dependent activation and CD40-induced expression of CD23, CD54, and CD150. Addition of exogenous LTB4 reversed the effect of the inhibitors. This study shows that LTB4 plays an important role in the activation of B-CLL cells. Inhibitors of leukotriene synthesis may be useful for the treatment of B-CLL.