Total loss of MHC class I in colorectal tumors can be explained by two molecular pathways: β₂‐microglobulin inactivation in MSI‐positive tumors and LMP7/TAP2 downregulation in MSI‐negative tumors

Abstract

The mechanisms that lead to loss of MHC class I expression in different types of tumors are not yet fully known. Accordingly, we studied colorectal carcinomas to elucidate the specific mechanisms of evasion of the T-cell immune response. We selected tumors with total loss of MHC class I expression and studied 124 colorectal carcinomas with immunohistochemical staining and anti-HLA monoclonal antibodies (mAb). Fourteen of 124 (11%) tumors exhibited a phenotype with HLA class I total loss. Microsatellite instability (MSI) analysis was also carried out in the same tumor samples. The expression of beta2-microglobulin (beta2m), HLA-A, B, and C antigens, transporter associated with antigen processing 1 (TAP1), TAP2, low-molecular-weight protein 2 (LMP2), and LMP7 were analyzed using reverse-transcription polymerase chain reaction (RT-PCR) in microdissected tumor samples. Four of 14 microsatellite instability-positive (MSI+) and W6/32 mAb-negative tumors showed biallelic inactivation of beta2m and accumulation of HLA class I heavy chain in the cytoplasm. MSI-negative (MSI-)/W6/32 mAb-negative tumors presented alterations in the expression of components of the antigen processing machinery (APM). Nine of 10 tumor samples showed LMP7 gene downregulation, and four of 10 presented TAP2 dysregulation. This group apparently expressed normal levels of heavy chain and beta2m mRNA. Two major mechanisms in colorectal cancer appear to be responsible for the total loss of MHC surface expression (beta2m mutations and LMP7/TAP2 downregulation) that may contribute to the failure of T lymphocyte recognition during an immune response. The precise identification of the molecular defects that underlie HLA class I abnormalities will have important implications for patients receiving T-cell-based specific immunotherapy.

The mutation in the ATP-binding region of JAK1, identified in human uterine leiomyosarcomas, results in defective interferon-γ inducibility of TAP1 and LMP2

“…The downregulation of MHC expression, including the TAP1 and LMP2 genes, is one of the biological mechanisms tumor cells use to evade host immunosurveillance (Singal et al, 1996;Dovhey et al, 2000;Cabrera et al, 2003). Recently, the incidence of IFN-g unresponsiveness in human tumors was examined in several cancers, and revealed that approximately 33% of each group exhibited a reduction in IFN-g sensitivity (Kaplan et al, 1998).…”

“…Total RNA was prepared from 5 Â 10 6 cells using TRIzol reagent according to the manufacturer's protocol (Invitrogen Co., CA, USA). The RNA was reverse-transcribed with the Superscript II enzyme (Invitrogen Co., CA, USA), the single-strand cDNA was used to amplify TAP1, LMP2, b2-m and b-actin transcripts using PCR analysis with the appropriate primer sets following a program of 35 cycles of 941C for 30 s, 601C for 30 s and 721C for 1.5 min with an additional 5 min for the extention of transcripts (Cabrera et al, 2003;Miyagi et al, 2003).…”

“…Many tumor cells unfortunately have lost this ability, thereby evading CTL-mediated immune surveillance and elimination (Garrido et al, 1997;Seliger et al, 2000;Dunn et al, 2002;Miyagi et al, 2003;Atkins et al, 2004). The defective antigen presentation by MHC class I is attributed to the downregulation of several genes required for antigen processing or presentation, such as TAP1 and TAP2, and LMP2 and LMP7 (Singal et al, 1996;Dovhey et al, 2000;Cabrera et al, 2003). Inactivation of the TAP1/LMP2 genes has been demonstrated in multiple distinct tumor types, and is associated with malignant transformation and disease progression (Dovhey et al, 2000).…”

Immunosurveillance is active in colorectal cancer as downregulation but not complete loss of MHC class I expression correlates with a poor prognosis

“…No attempt was made to score cytoplasmic staining with HC10, as it has been observed that the antibody defines free HLA class I heavy chain and thus may show a positive reaction in the cytoplasm in tumors with W6/32 negativity and no surface MHC class I expression. 14 In the case of b 2 -microglobulin staining, tumors were categorised as either b 2 -m negative (complete absence of tumor cell staining) or b 2 -m positive. Where discrepancies arose between the staining of cores from the same tumor, an average of the evaluable cores was taken, with confirmation by 2 observers (Z.M.…”

High density of FOXP3-positive T cells infiltrating colorectal cancers with microsatellite instability

“…For example, immune evasion mechanisms such as impairment or loss of the human leukocyte antigen class I-mediated antigen presentation (Bicknell et al, 1996;Cabrera et al, 2003;Kloor et al, 2005) or the expression of Fas ligand (Okada et al, 2000;Michael-Robinson et al, 2003) are frequent in MSI-H CRCs.…”