Teaching the Immune System to See Tuberculosis: My PhD Research on the M. tuberculosis Surface

My PhD research, conducted in Dr. Arturo Casadevall's laboratory, focused on a single molecule on the surface of Mycobacterium tuberculosis, the bacterium that causes tuberculosis, and a question that sounds simple but drives a huge amount of vaccine science: is this molecule actually a good target for the immune system, and how would we know? Answering that question requires separating two ideas that get casually lumped together outside immunology — antigenicity and immunogenicity — because a molecule can have a great deal of one and very little of the other.

Antigenicity and immunogenicity are not the same thing

An antigen is any molecule that can be specifically recognized and bound by an antibody or a T-cell receptor. Antigenicity describes how well a molecule is recognized and bound once an antibody against it already exists — essentially, how good a target it makes. Immunogenicity describes something different: how effectively that same molecule provokes the immune system to produce antibodies against it in the first place. These can diverge sharply. A molecule can be extremely antigenic — bound tightly and specifically by an antibody in a test tube — while being poorly immunogenic, meaning a real immune system, encountering it during an actual infection or vaccination, mounts only a weak or short-lived response against it. This distinction is the difference between "a good target" and "a target the immune system will actually learn to attack on its own," and it sits at the center of essentially every rational vaccine design problem.

The molecule: arabinomannan, a carbohydrate on the bacterial surface

The molecule at the center of my research was arabinomannan (AM), a complex polysaccharide (built from arabinose and mannose sugar units) that is abundantly present on the M. tuberculosis cell surface as part of a larger structure called lipoarabinomannan. AM posed exactly the antigenicity/ immunogenicity problem described above: polysaccharide antigens are generally recognized as T-independent antigens, meaning they can trigger antibody production without engaging T-helper cells — but T-independent responses are typically weaker, less durable, and don't generate the long-term immune memory that a strong, T-cell-engaged (T-dependent) response, typical of protein antigens, produces. Establishing whether AM was even reliably present — not just in a lab culture dish, but during a real, active infection — was a necessary first step before anyone could seriously evaluate it as a vaccine or diagnostic target.

What the research actually showed

My study, published in Infection and Immunity in 2001 with Arturo Casadevall and colleagues, used a specific monoclonal antibody (designated MAb 9d8) engineered to bind AM, and applied it to directly test whether AM is expressed both in vitro (in cultured M. tuberculosis) and in vivo (during actual infection). This mattered because a molecule's presence in a petri dish culture doesn't guarantee it is actually displayed, in a recognizable form, on bacteria during a real infection inside a host — and a vaccine or diagnostic strategy built around a molecule that disappears or changes form in vivo would be built on a false assumption. Confirming consistent AM expression in both settings was the necessary evidence establishing that AM is a legitimate, stable target worth pursuing further — the antigenicity-and-immunogenicity question applied to one specific, real molecule rather than treated as an abstract concept.

Why this connects to real vaccines: the conjugate vaccine strategy

This kind of finding matters because of an established, powerful strategy in vaccine design: when a polysaccharide antigen is poorly immunogenic on its own, chemists can chemically link it to a strong protein carrier, creating a conjugate vaccine. The attached protein recruits T-helper cells that the polysaccharide alone could not engage, converting a weak, short-lived T-independent response into a much stronger, longer-lasting, memory-forming T-dependent one. This exact strategy underlies several vaccines already in routine use — the Hib (Haemophilus influenzae type b), pneumococcal, and meningococcal conjugate vaccines all work by pairing a poorly immunogenic bacterial polysaccharide with a protein carrier. Research establishing that AM is a real, stable, in-vivo-expressed antigen on M. tuberculosis is a piece of the foundation needed to seriously evaluate whether a similar AM-conjugate approach could someday improve on BCG, the current, imperfect century-old TB vaccine, or contribute to better antibody-based TB diagnostics.

Antigen

Any molecule that can be specifically recognized and bound by an antibody or T-cell receptor.

Antigenicity

How well a molecule is recognized and bound by an antibody once that antibody exists; a measure of how good a target the molecule makes.

Immunogenicity

How effectively a molecule provokes the immune system to produce an antibody response against it in the first place.

T-independent antigen

An antigen, typically a polysaccharide, that can trigger antibody production without engaging T-helper cells, generally producing a weaker and less durable response than a T-dependent (typically protein) antigen.

Conjugate vaccine

A vaccine strategy that chemically links a poorly immunogenic polysaccharide antigen to a strong protein carrier, recruiting T-helper cells to produce a stronger, longer-lasting immune response.

Check your understanding

Sources: Schwebach JR, Casadevall A, Schneerson R, Dai Z, Wang X, Robbins JB, Glatman-Freedman A, "Expression of a Mycobacterium tuberculosis arabinomannan antigen in vitro and in vivo," Infection and Immunity, September 2001, 69(9):5671–8; standard immunology references on antigenicity, immunogenicity, T-dependent vs. T-independent antigens, and conjugate vaccine design (Hib, pneumococcal, meningococcal vaccines). DRAFT — verify current 2018 Virginia Science Standards of Learning biology codes with the current Curriculum Framework before publishing; this reading covers the author's own published research and should be reviewed for accuracy and appropriate framing for a student audience.