Host-Adapted Gene Families Involved in Murine Cytomegalovirus Immune Evasion

Abstract

Cytomegaloviruses (CMVs) are host species-specific and have adapted to their respective mammalian hosts during co-evolution. Host-adaptation is reflected by "private genes" that have specialized in mediating virus-host interplay and have no sequence homologs in other CMV species, although biological convergence has led to analogous protein functions. They are mostly organized in gene families evolved by gene duplications and subsequent mutations. The host immune response to infection, both the innate and the adaptive immune response, is a driver of viral evolution, resulting in the acquisition of viral immune evasion proteins encoded by private gene families. As the analysis of the medically relevant human cytomegalovirus by clinical investigation in the infected human host cannot make use of designed virus and host mutagenesis, the mouse model based on murine cytomegalovirus (mCMV) has become a versatile animal model to study basic principles of in vivo virus-host interplay. Focusing on the immune evasion of the adaptive immune response by CD8⁺ T cells, we review here what is known about proteins of two private gene families of mCMV, the m02 and the m145 families, specifically the role of m04, m06, and m152 in viral antigen presentation during acute and latent infection.

Keywords: CD8 T cells; adoptive cell transfer; antigen presentation; antiviral protection; co-evolution; cytomegalovirus; gene family; immune evasion; latent infection/latency; memory inflation.

Figure 1

Map positions of mCMV gene families m02 and m145. Arrows indicate the direction of transcription on the coding DNA strand [20,21]. In mCMV gene nomenclature, lower case “m” indicates an absence of homology to hCMV genes.

Figure 2

High evolutionary conservation of m04 and m06 cargo sorting motifs in strains of mCMV. (Smith NC_004065 [20], K181 AM886412 [42], WP15B EU579860 [43], G4 EU579859 [43], N1 HE610454 [44], C4A EU579861 [43], C4B HE610452 [44], C4C HE610453 [44], C4D HE610456 [44], s02 MH118557 [45], s09 MH118555 [45], s17 MH118558 [45], s45 MH118556 [45], s88 MG957497 [45], NO7 HE610455 [44], AA18d HE610451 [44], WT1 GU305914 [46]).

Figure 3

(A) Alternative models of viral gene expression during latency. (Circle) latent viral genome in the nucleus of a latently infected cell. (Antigen) IE1 and/or m164 peptide; (vRAP) m06 and/or m152; (MI) memory inflation; (iTEM) inflationary T effector-memory CD8⁺ T cell. (B) BALB/c (H-2^d) HCT model for establishing mCMV latency in the lungs. For the analysis of transcripts by RT-PCR, lungs are cut into 18 pieces, p1-p18; (SL, ML, IL) superior, middle, and inferior lobe of the right lung; (PCL) post-caval lobe; (LL) left lung. (C) Experimentally observed random transcription patterns in 5 mice (#1–#5) tested individually. Reproduced from reference [95] in a new arrangement.

Figure 4

(A) Adoptive transfer of M45-D^b CTL into immunocompromised C57BL/6 mice infected with viruses mCMV-Δm04m06, selectively expressing vRAP m152, mCMV-Δm06 expressing vRAPs m152 and m04, as well as wild-type (WT) mCMV expressing vRAPs m152, m04, and m06. Data represent infected m152⁺ cells present in 50-mm² areas of liver tissue sections. The red dot symbol represents mice tested individually. Median values are marked and connected for comparison. (A-arrow up or down), positive or negative effect on antigen presentation, respectively; (AT) adoptive transfer. P values for comparing groups with or without AT were calculated by a Student’s t-test with Welch’s correction for unequal variances. Differences are considered significant for P < 0.05. The dashed line represents the detection limit of the assay. Note that the lower viral load in the “no AT” group of infection with mCMV-Δm06 does not reflect virus attenuation but results from a lower initial dose of infection. Reproduced from [32] in a new arrangement. (B) Representative images of liver tissue sections stained by 2-color immunohistochemistry and hematoxylin counter-staining. (Red) Infected liver cells, mostly hepatocytes, identified by a cytoplasmic expression of m152 that is shared by all three viruses; (Black) Tissue-infiltrating CD8⁺ T cells. Bar marker: 50 μm. Reproduced from reference [32] in a new marker combination.

Figure 5

Graphical abstracts explaining agonistic and antagonistic vRAP functions. (ER), endoplasmic reticulum; (ERGIC), ER Golgi-intermediate compartment; (TGN), trans-Golgi network; (ERC), endosomal recycling compartment. Note that an early gene function of mCMV blocks cargo recycling from the ERC to the cell surface [103]; (EE), early endosome; (LE), Late endosome; AP, cellular adapter protein. Solid arrows indicate main pathways; the dashed arrow indicates an alternative route.