Preface.

Howard E. Gendelman

There is no abstract to the preface, however the first paragraph is presented instead. This unique offering provides both the novice and experienced reader with a "state of the art" in-depth insight into the biology and pathogenesis of lentiviral induced disease that occurs following infection of mononuclear phagocytes (MP; blood borne macrophages, tissue histiocytes, dendritic cells, and microglia). This book, as no other, is the sole and most comprehensive scholarly writing to date covering the pivotal role played by these MP as reservoirs and vehicles for pathogen dissemination and in disease induction. For chronic lentiviral infections MP remain an enigma. On the one hand they are among the first cells contacted by virus and despite a virtual armada of immunological tools, still serve as means to both spread and contain infection. Virus can simultaneously assemble and hide in intracellular compartments, largely free from immune attack. Interestingly, the MP are not destroyed by virus and throughout infection they still contribute to host immunity while at the same time perpetuating lentiviral dissemination. Infected MP are readily observed in lymph nodes and organs such as the lung and brain, where they produce cytotoxic mediators that contribute to the development of disease. For human immunodeficiency virus (HIV) infection functional impairment of infected MP likely accelerates immune deficiency. Thus, the questions most asked are how can a cell that possesses so many intrinsic defense mechanisms harbor such viral pathogens for prolonged time periods? How can MP serve both as sentries and vehicles for disseminating infection and inducing disease? Indeed, in regards to biology, both for ontogeny and phylogeny, MP are the most primitive sensors of tissue injury and as such serve to clear debris and simultaneously protect the host's homeostatic environment. Their roles serve the host in non-specific defense (innate immunity) and in initiating cell-specific protection (adaptive immunity). A consistent evolutionary role resides in the ability of MP to engulf, digest and destroy cell and tissue debris through phagolysosomal fusion. Interestingly, MP can affect neighboring lymphocytes and other immunocytes to perform similar functions, albeit by divergent mechanisms. Control of viral growth occurs together with the MP's notable possession of a vast repertoire of immune secretory factors that include pro-inflammatory cytokines, chemokines, arachidonic acid and its metabolites, platelet activating factor, nitric oxide, quinolinic acid, amongst others. These serve to regulate immune defense, cell mobility, antigen presentation, immune activation, and cell differentiation. During disease such secretions are induced by infection and affect inflammatory processes that speed cell and tissue injury leading to clinical symptoms and morbidities. This, in the case of common lentiviral tissue injuries, leads to substantive lung, brain, blood, and joint diseases. Clearly, the role played by MP in the pathogenesis of lentiviral infections is seemingly complex and quite multifaceted. Viral spread from MP to MP and from MP to T cells and across cell and tissue barriers are equally vast and complicated.

1. Lentivirus Tropism and Disease

Jodi K. Craigo and Ronald C. Montelaro

Lentiviruses are among the most intensely and extensively studied group of viruses. They are found worldwide and infect a broad array of animal species. Historically, lentiviruses have been investigated longer than any other virus group. The first viral etiology ascribed to an animal disease was a lentivirus. The diseases associated with lentiviral infections range from benign and subclinical to severely debilitating and lethal. The diverse group of viruses that compose the lentiviruses have many common and distinctive features. Among the common features is tropism for cells from the monocyte/macrophage lineage. Infection of macrophage affords this assorted group of viruses many evolutionary advantages, including a potential hiding place from the infected host's immune system.

2. A Bird's Eye View of Macrophage Biology

Ian Ross

Macrophages possess elaborate sensors for pathogens and have evolved complex chemically mediated interactions with other components of the immune system. Macrophage precursors differentiate into multiple end-stage specialised cell types in mammals, each suited for specific roles in homeostasis and defence. This chapter attempts to paint a broad picture of macrophage biology, ranging from embryological origins, through their role in the coordination of immune defences, to specific mechanisms used to address different kinds of threats to the parent organism. Research in macrophage biology is at a particularly exciting stage with new defence pathways still in the process of discovery. At the same time, enough is known to glimpse the complete picture that will one day be available for this staggeringly complex molecular machine which is a central organiser of immune defences. Macrophages are critically involved in disease in humans and animals because of their roles both in chronic inflammatory diseases, and during infections by important pathogens, including lentiviruses. In each case, an understanding of macrophage defensive mechanisms and how they have been maladapted or thwarted in disease states provides a range of opportunities for specific therapeutic intervention.

3. The Macrophage in HIV-1 Infection and Disease Progression

Paul R. Gorry, Jasminka Sterjovski and Melissa J. Churchill

Rapid depletion of CD4+ T-lymphocytes has been associated with a switch in HIV-1 coreceptor usage from CCR5 to CXCR4 in approximately 40 to 50% of infected individuals. However, the majority of infected individuals who progress to AIDS harbour only CCR5-dependent (R5) viral strains. HIV-1 disease progression is associated with an enhanced tropism of R5 viral strains for cells of the monocyte/macrophage lineage (enhanced M-tropism). However, the underlying molecular mechanisms contributing to enhanced M-tropism by R5 HIV-1 strains, and how HIV-1 variants with enhanced M-tropism cause CD4+ T-cell depletion in vivo are unknown. This chapter examines the relationship between viral coreceptor usage, M-tropism, and pathogenicity of HIV-1. We highlight evidence supporting the hypothesis that enhanced M-tropism of R5 HIV-1 may result from adaptive viral evolution, resulting in HIV-1 variants that have increased ability to utilize relatively low levels of CD4 and CCR5 expressed on macrophages. The evidence also suggests that these late-emerging, R5 viral strains have reduced sensitivity to entry inhibitors, and increased ability to cause CD4+ T-lymphocyte loss. These variants are likely to impact HIV-1 disease progression, particularly in patients who persistently harbour only R5 viral strains.

4. Post-entry Restrictions to Lentiviral Replication

Jenny L Anderson and Gilda Tachedjian

Host organisms contain numerous defenses to protect themselves against invading pathogens like lentiviruses. These defense systems include intracellular inhibitory proteins termed "restriction factors", which have recently risen to prominence with the discovery of APOBEC3 and TRIM5 cellular proteins as key factors restricting invading retroviruses in host cells. Understanding how these and other intracellular restriction factors block retroviruses is providing new insights into barriers of retroviral replication and the cross-species transmission of lentiviruses. Moreover, these restriction factors have important ramifications for identifying new targets for antiviral therapy, developing better models of HIV-1/AIDS and selecting retroviral vectors for gene therapy. Thus, restriction factors have emerged as a dynamic, important area of lentivirus research. This chapter reviews rapid advances in understanding intracellular restriction factors targeting retroviruses after entry into host cells. These restriction factors include TRIM5α, TRIMCyp, Cyclophilin A, Fv1, APOBEC3, TRIM28, ZAP, tetherin and calcium-modulating cyclophilin ligand. Lentiviral countermeasures to circumvent these intracellular restrictions are also canvassed. Delineating the biology of these intracellular restrictions appears promising for developing novel antiviral therapies to curb lentiviruses as effectively as the naturally occurring intracellular restriction factors.

5. HIV-2 Tropism and Disease

Kelly Cheney and Áine McKnight

Human immunodeficiency viruses (HIV-1 and HIV-2) have evolved from a reservoir of African non-human primate lentiviruses, the simian immunodeficiency viruses. In contrast to the epidemic nature of HIV-1 infections, HIV-2 is restricted in its worldwide distribution, with the lower viral loads established in asymptomatic infection a significant cause of its diminished transmission efficiency. HIV-2 is also much less pathogenic than HIV-1 and results in a reduced rate of progression to AIDS despite a substantial proviral burden. The majority of patients remain asymptomatic and die of causes unrelated to immunodeficiency.

The adoption of "accessory genes" by HIV-2 and its more promiscuous pattern of coreceptor usage (including CD4-independence) may assist the virus in its adaptation to avoid innate restriction factors present in host cells. Adaptation to use normal cellular machinery to enable transmission and productive infection has also aided the establishment of HIV-2 replication in humans. A survival strategy for any infectious agent is not to kill its host but ultimately become a commensal organism. Having achieved a low pathogenicity, over time, variants more successful at transmission will be selected.

6. SHIV Model of Disease

Tatsuhiko Igarashi

Simian-human immunodeficiency virus (SHIV) was generated as an animal model for acquired immunodeficiency syndrome (AIDS) in order to overcome the narrow host range of human immunodeficiency virus (HIV-1). The first-generation SHIVs were nonpathogenic but evolved through animal-to-animal passage to become highly pathogenic viruses. In macaque monkeys, highly pathogenic SHIVs induce a distinct disease phenotype: a massive, systemic, and irreversible depletion of CD4+ T cells occurs within weeks of infection, followed by AIDS-like clinical manifestations. During the acute phase of infection, the virus predominantly infects and destroys CD4+ T cells. As a result, macrophages become the major virus-producing cell type. Virus isolated during the macrophage phase of infection exhibits macrophage tropism, a property not possessed by the inoculum SHIV. The V1/V2 region of the env gene was found to be responsible for the expanded cell tropism observed in the macrophage-tropic virus. The viral entry coreceptor, CXCR4, was maintained during the evolution of the virus. The vast majority of macrophage -tropic viruses are attenuated when inoculated into immune-competent monkeys.

7. SIV Pathogenic and Nonpathogenic Infections

Thaidra Gaufin Ivona Pandrea and Cristian Apetrei

Simian immunodeficiency viruses (SIVs) naturally infect African nonhuman primates. Cross-species transmissions of SIVs from naturally infected chimpanzees/gorillas and sooty mangabeys are at the origin of HIV-1 and HIV-2, respectively. Experimental or accidental transmission of SIVsmm to different species of macaques resulted in the development of AIDS animal models. Differently from humans and macaques which, upon infection, invariably progress to AIDS, natural SIV hosts are generally spared of disease progression. Pathogenic and nonpathogenic SIV infections share some major features, such as a very active viral replication during both acute and chronic infection, a significant acute depletion of CD4+ T cells, which is more prominent at mucosal sites, and a partial control of the virus by both adaptive and innate immune responses. Although SIVs have the potential to infect macrophages, the bulk of viral replication in vivo is supported by CD4+ T cells in both pathogenic and nonpathogenic infections. The major differences between the two models are: maintainance of CD4+ T cell homeostasis in natural hosts, with rebounds to near preinfection levels of peripheral CD4+ T cells and significant recovery in the intestine; normal levels of T cell immune activation, cell proliferation and apoptosis in natural infection, while increases in these three parameters are associated with disease progression in pathogenic infection. A better understanding of the mechanisms underlying the lack of disease in natural hosts for SIV infection will likely provide important clues as to the pathogenesis of AIDS in HIV-infected individuals.

8. The Felid Immunodeficiency Viruses: Viral Cell Tropism and the Pathogenesis of Feline AIDS.

Brian J. Willett and Margaret J. Hosie

Lentiviruses are widespread pathogens of primates, ungulates and felids. While the ungulate lentiviruses induce a disease state typical of a chronic inflammatory condition, the felid and primate lentiviruses induce an immunodeficiency characterised by a progressive depletion of CD4+ T helper cells. FIV infection of the domestic cat may lead to a spectrum of diseases, ranging from a rapid, acute-onset immunodeficiency to a chronic wasting disease with concomitant neuropathology and persistent recurring opportunistic infections. Here, we examine the host and viral determinants of FIV cell tropism and pathogenicity. The virus targets activated CD4+ T cells selectively by interactions with its primary receptor, CD134 and co-receptor, CXCR4. We discuss the impact of the virus-receptor interaction on the normal cellular function of the viral receptors and the cells on which they are expressed and assess how this contributes to pathogenicity, immunity to infection and the prospects for the development of FIV vaccines.

9. Equine Infectious Anemia Virus Pathogenesis and Replication

Wendy Maury and J. Lindsay Oaks

Equine infectious anemia virus (EIAV) is an ungulate lentivirus related to human immunodeficiency virus (HIV). Much of the understanding of lentiviral infection of macrophages comes from HIV studies that have provided insights into molecular regulation of all lentiviruses. However, numerous aspects of the life cycle of each lentivirus are unique and associated with specific pathological consequences. In vivo EIAV is primarily if not exclusively a macrophage-tropic virus. As a consequence of this targeted tropism, EIAV causes an acute and sometimes fulminant disease associated with high-titered viremia with no associated immunodeficiency. Investigations have only begun to unravel the molecular mechanisms leading to cell-specific replication of EIAV.

10. Small Ruminant Lentiviruses and Cross Species Transmission

Giuseppe Bertoni and Barbara Blacklaws

The Visna-Maedi virus (VMV) and the caprine arthritis encephalitis virus (CAEV) were considered to be specific pathogens of sheep and goats, respectively. The finding that these lentiviruses frequently cross the species barrier between sheep and goats, and vice versa, has changed our view of the epidemiology of these viruses and they are now referred to as small ruminant lentiviruses (SRLV). A brief review of the molecular epidemiology of these lentiviruses will illustrate the diffusion and intermixing of these viruses in the two target species and documented cases of double infection and recombination between VMV and CAEV will be discussed.

Monocytes-macrophages and dendritic cells are the main target cells of CAEV. Monocytes carrying the lentiviral provirus in their genome show little or no viral transcription. These latently infected cells are believed to function as "Trojan horses", capable of spreading the virus to different organs, while eluding the host immune response. The terminal maturation of monocytes to macrophages activates the expression of the transcription factors that, by interacting with the control elements in the viral LTRs, promote the production of infectious virus. The LTR sequences of different SRLV isolates are quite heterogeneous and may control the tissue-specific replication of these viruses and their virulence. SRLV replicate unrestrictedly and to high titers in differentiated macrophages in vitro, whereas in vivo virus replication is tightly controlled by mechanisms involving innate immunity and the adaptive immune system, and the intrinsic resistance of cells to retrovirus replication.

SRLV manipulate the expression of different cytokines in infected cells and modulate the cytokine response of these cells to stimulation of the various receptors involved in recognizing pathogen associated molecular patterns (PAMPS). The genetic background of animals influences the clinical outcome of SRLV infection which, in contrast to HIV infected humans, is mainly benign in the majority of infected animals. Most animals therefore fulfill the criteria defining long-term non-progressors. Finally, the various strategies adopted by SRLV to manipulate the aforementioned immunological and non-immunological antiviral mechanisms directly influence the efficacy of vaccination strategies as documented by the paradoxical effects induced by experimental vaccines on viral load and pathological manifestations in vaccinated and challenged animals.

11. The Bovine Lentiviruses: Pathogenesis and Cell Tropism

Moira Desport and Sandy McLachlan

Infections with the bovine lentiviruses, bovine immunodeficiency virus (BIV) or Jembrana disease virus (JDV) represent the extremes of lentivirus induced disease. BIV has a broad cell tropism and causes a mild lymphoproliferative disorder with low viral titres and no reproducible disease sequelae. JDV has a more restricted cell tropism than BIV and infects Bali cattle in Indonesia, replicating to high viral titres during an acute disease period characterized by lymph node enlargement, leucopaenia and high rectal temperatures. The similarities and differences between these two genetically and antigenically closely related viruses and between other lentiviruses will be reviewed in this chapter with particular regard to their cell tropism, pathogenesis and genetic composition.

12. Lentivirus Coinfections and Superinfections

Sue VandeWoude and Mary Poss

The phenomenon of lentiviral superinfection, that is infection with a second unique strain of virus after a primary viremia has already been established, is an important aspect of lentivirus biology, and is being increasingly recognized as a method for rapid emergence of new strains via the process of recombination. Though there are both host and viral barriers to simultaneous coinfection following exposure with two unique lentiviruses, it is well established in both natural and experimental systems that infection with one strain of lentivirus does not impart resistance to superinfection. Viral kinetics and dynamics of infection are altered in coinfections relative to single infections. Superinfection may result in either diminished or enhanced virulence of the primary or secondary strain and may occur with high prevalence in individuals with repeated exposures. Evaluation of the incidence of naturally occurring superinfections, and biological behavior of experimental superinfections, either with host-adapted or non-host-adapted strains, provides insights into potential pitfalls and additional strategies to consider during lentiviral vaccine development. Analysis of lentiviral genome recombination events as a sequelae of superinfection enhances understanding of mechanisms of lentiviral evolution and virus-host adaptations. This chapter provides a review and comparison of lentiviral superinfections and resultant recombination outcomes in different hosts, including primate, felidae, and ruminant species, highlighting what these studies have revealed about lentiviral pathogenesis, host immune response to lentiviral infection, and viral genomic responses to superinfections.