**4. Cellular factors with inhibitory activity on HIV replication and implications in viral pathogenesis**

Innate immunity had evolved as a mechanism to defend eukaryotes from bacterial and viral infections. These mechanisms rely on different cellular restriction factors that suppress the replication of the pathogens, namely retroviruses [22].

During HIV-1 infection, incoming viral RNA triggers a TLR7/8-mediated innate immune response, resulting in the production of type I interferon (IFN). In particular IFNα has been shown to be up-regulated after TLR sensing during acute infection with HIV-1 or SIV [23-25]. Accordingly, initial observations *in vitro* revealed that pre-treatment of macrophages with type-I IFN inhibited the replication of HIV-1, indicating that potent inhibitory factors were induced after IFN exposure [26, 27]. Most of them are still uncharacterized.

The identification of cellular restriction factors and the viral proteins that antagonize those restrictions have stimulated an active area of research that explores crucial mechanisms underlying HIV interference with cellular restriction factors and innate immunity. In this subchapter specific cellular factors with inhibitory activity on HIV replication are discussed including how viral-encoded proteins counteract these factors.

#### **4.1. TRIM5α**

The search for the mechanisms underlying the innate cellular resistance to retroviral infections shown by different non-human primate species, has led to the identification of a cytoplasmic factor that prevented infection of Old World monkeys by HIV-1 [28]. This factor – TRIM5α – was identified as a member of the tripartite motif (TRIM) family of proteins, a large family of cellular proteins with distinct biological activities including innate immune signaling [29]. After its initial identification in rhesus macaques (rhTRIM5α) [28] and owl monkeys (TRIM‐ Cyp) [30], TRIM5α was also identified as a retroviral restriction factor in humans [31, 32] that is induced by both type I and type II IFN [33].

Different models have been proposed for retroviral inhibition mediated by TRIM5 proteins [34]. They suggest that these proteins mediate restriction by directly binding to specific determinants in the viral CA protein, blocking HIV replication soon after viral release in host cell cytoplasm. The TRIM proteins family is defined by three domains (RING, B-Box2, and Coiled- Coil), which are present in all members of this family. The N-terminal RING domain possesses E3 ubiquitin ligase activity that is crucial for retrovirus restriction [35, 36]. The B-Box2 and Coiled Coil (CC) domains are thought to contribute to the higher and low order multimerization of TRIM5α, respectively. The TRIM5α also possesses a C-terminal capsid binding domain that mediates specific recognition and restriction of certain retroviruses [37]. The recognition of viral capsid determinants (CA protein) relies on three variable regions present in the C-terminal domain of TRIM5α, and apparently they are equally involved in retrovirus recognition and restriction [38-41].

Several studies have addressed the mechanisms by which TRIM5α protein prevents viral infection and different models have been proposed to explain this restriction. The "accelerated uncoating" model was based on the observation that cytosolic CA protein was specifically dissociated in rhTRIM5α-expressing cells [42] leading to the proposal of a "proteasome independent capsid degradation" mechanism. This model suggests that the stripping of capsid protein prevents viral RTC to proceed to subsequent steps in infectious replication cycle, namely the reverse transcription and nuclear import [42]. An alternative model was primarily based on the observation that proteasome inhibitors allows reverse transcription and integra‐ tion, without affecting the TRIM5α-mediated restriction [43, 44]. Accordingly, a "two-step restriction mechanism" was proposed, suggesting that restriction activity of TRIM5α occurs by both proteasome-dependent and -independent pathways. The relative contribution of each pathway is apparently dependent on host cells-viruses combinations [45].
