Blockade of the granzyme B/perforin pathway through overexpression of the serine protease inhibitor PI-9/SPI-6 constitutes a mechanism for immune escape by tumors

Abstract

The concept for cellular immunotherapy of solid tumors relies heavily on the capacity of class I MHC-restricted cytotoxic T lymphocytes (CTLs) to eliminate tumor cells. However, tumors often have managed to escape from the cytolytic machinery of these effector cells. Therefore, it is very important to chart the mechanisms through which this escape can occur. Target-cell killing by CTLs involves the induction of apoptosis by two major mechanisms: through death receptors and the perforin͞granzyme B (GrB) pathway. Whereas tumors previously were shown to exhibit mechanisms for blocking the death receptor pathway, we now demonstrate that they also can resist CTL-mediated killing through interference with the perforin͞GrB pathway. This escape mechanism involves expression of the serine protease inhibitor PI-9͞SPI-6, which inactivates the apoptotic effector molecule GrB. Expression of PI-9 was observed in a variety of human and murine tumors. Moreover, we show that, indeed, expression results in the resistance of tumor cells to CTL-mediated killing both in vitro and in vivo. Our data reveal that PI-9͞SPI-6 is an important parameter determining the success of T cell-based immunotherapeutic modalities against cancer.

Complete loss of HLA class I antigen expression on melanoma cells: A result of successive mutational events

“…Tumor escape mechanisms that alter both functions have been described: (i) downregulation of Fas death receptors and defects in death receptor signaling 28 and (ii) blockade of the perforin pathway through overexpression of serine protease inhibitors. 29 In addition, these evasion strategies can be enforced by simultaneous secretion of immunosuppressive cytokines as well as downregulation of tumor antigen expression. 30 -34 The information on immune evasion strategies developed by tumors clearly indicates that cancer cells successively acquire various mutations to survive, disseminate and metastasize in an immunocompetent host.…”

Death by a Thousand Cuts: Granzyme Pathways of Programmed Cell Death

“…SPI-6 is expressed in CTL and NK cells (103) and is upregulated during DC maturation (104). Overexpression of SPI-6 in target cells protects them from CTL killing (105). In SPI-6 transgenic mice, increased numbers of CTLs persist long after viral clearance, suggesting that SPI-6 protects CTLs from self-destruction (103).…”

“…Early studies with solid tumors and lymphomas in humans and mice suggested that overexpression of PI-9 or SPI-6 may be a mechanism by which tumors evade the GzmB/ perforin pathway (105,107). In these studies there is no comparison of serpin expression in tumor cells relative to corresponding normal tissues, thus making the results difficult to interpret.…”

Identification of a naturally processed HLA‐A*0201 HPV18 E7 T cell epitope by tumor cell mediated in vitro vaccination

“…In earlier studies with HeLa-specific allogeneic T-cell lines and at higher E:T ratios, HeLa cells could not be lysed at significantly higher rates. 25,50 The identification of the new tumor relevant CTL epitope HPV18 E7 86 -94 demonstrates the power of the epitope-finding strategy that we present here. The advantage of our method over conventional strategies is its inherence of tumor relevance of the identified epitopes.…”

“…In this library, consequently, all possible epitope sequences of the HPV18 E7 were represented. Sequences are: HPV18 E7 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] , p44 (MHGP-KATLQDIVLHLEPQNE); HPV18 E7 [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] , p45 (VLHLEPQNE-IPVDLLCHEQL); HPV18 E7 , p46 (VDLLCHEQLSDSEEEN-DEID); HPV18 E7 34 -53 , p47 (SEEENDEIDGVNHQHLPARR); HPV18 E7 [45][46][47][48][49][50][51][52][53]…”