A 28-year-old woman from Mumbai, diagnosed with HIV-1 infection 6 months ago, presents with a CD4+ count of 45 cells/μL despite being on antiretroviral therapy (ART) for 4 months. Viral load is undetectable (<50 copies/mL). She develops acute confusion, headache, and fever. CSF analysis shows elevated protein (180 mg/dL), low glucose (25 mg/dL), and negative bacterial culture. India ink stain of CSF is positive. Which structural component of HIV-1 is responsible for the initial attachment and entry into CD4+ T cells, and how does its loss relate to her current opportunistic infection?

Explanation

## HIV-1 Attachment, Entry, and Immunosuppression ### HIV-1 Entry Mechanism **Key Point:** gp120 is the surface glycoprotein that binds to CD4 receptors on target cells; gp41 is the transmembrane protein that mediates fusion. Together they form the envelope spike complex. ### Sequential Entry Process 1. **gp120-CD4 Binding (Attachment)** - gp120 recognizes and binds CD4 receptor on T cells, macrophages, dendritic cells - Conformational change exposes hidden epitopes - Allows access to co-receptors (CCR5 or CXCR4) 2. **gp41-Mediated Fusion** - gp41 inserts fusion peptide into target cell membrane - Hairpin formation brings viral and cellular membranes together - Membrane fusion and entry of viral core ### Why CD4+ Depletion Causes Opportunistic Infection **High-Yield:** The patient's CD4 count of 45 cells/μL (normal >500) indicates severe immunosuppression. *Cryptococcus neoformans* is an encapsulated yeast that requires intact cell-mediated immunity for control. With CD4 <50, risk of cryptococcal meningitis is ~10% annually without prophylaxis. ### Clinical Correlation: Cryptococcal Meningitis | Feature | Finding in This Patient | |---------|------------------------| | **CD4 threshold** | <50 cells/μL (highest risk) | | **CSF glucose** | Low (25 mg/dL) — fungal consumption | | **CSF protein** | Elevated (180 mg/dL) — inflammation | | **India ink stain** | Positive — visualizes capsule | | **Cryptococcal antigen** | Positive in serum and CSF | | **Bacterial culture** | Negative — rules out bacterial meningitis | **Clinical Pearl:** *Cryptococcus neoformans* is the most common cause of meningitis in advanced HIV (CD4 <50). The India ink stain shows the characteristic halo (capsule) around the yeast. Cryptococcal antigen testing (serum and CSF) is more sensitive and is the diagnostic standard. ### Relationship Between gp120 and Immunosuppression ```mermaid flowchart TD A[HIV-1 virion with gp120/gp41]:::outcome --> B[gp120 binds CD4 receptor]:::action B --> C[gp41 mediates fusion]:::action C --> D[Viral entry into CD4+ T cell]:::outcome D --> E[Viral replication]:::action E --> F[CD4+ T cell death]:::urgent F --> G[Progressive CD4+ depletion]:::urgent G --> H{CD4 count <50?}:::decision H -->|Yes| I[Loss of cell-mediated immunity]:::urgent I --> J[*Cryptococcus neoformans* dissemination]:::urgent J --> K[Cryptococcal meningitis]:::outcome ``` **Mnemonic:** **gp120-CD4-gp41** — Surface attachment → Co-receptor binding → Transmembrane fusion → Entry ### Why Undetectable Viral Load Does Not Prevent Opportunistic Infection Despite virological suppression (undetectable viral load), the patient has severe CD4+ depletion from prior untreated infection. CD4 recovery on ART takes months to years; immune reconstitution is delayed. During this window, opportunistic infections occur — this is called **immune reconstitution inflammatory syndrome (IRIS)** risk period. **Warning:** Do not confuse virological suppression with immunological recovery. CD4 count is the primary predictor of opportunistic infection risk, not viral load.