AIDS & mechanism of no granulomas

[maryjane85]

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Practice question from NBME practice exam today about a patient with AIDS, and why they don't form granulomas in response to tuberculosis. Answer choices are:
A) Clonal deletion of T lymphocytes reactive to TB antigen
B) Inability of Th1 cells to attract circulating monocytes
C) Defect in macrocyte phagocytosis.


My dilemma: had choice A not included the word "clonal" I would have no issues with this answer choice being correct but because they threw in clonal I'm not so sure. I know that macrophages would have decreased killing ability and decreased epithelioid due to the decrease in CD4 count in AIDS due to IFN gamma deficiency. IFN gamma and other CD4 products dont directly stimulate macrophages to phagocytose bacteria BUT it does stimulate antibodies which can bind to bacteria and stimulate macrophage phagocytosis.
I ended up putting C (I dont remember what the other two answer choices were but they were obviously wrong) but wanted to get other opinions....was I right in not choosing A because of the word "clonal"???

Thanks for any help! 1 week out from step 1 =)

 

[Ty Webb]

what was your rationale behind eliminating answer choice "B"?

 

[Fuzuli]

"Clonal deletion" is a specific term for positive and negative selection of T-cells, so I doubt they would've written it as "deletion". Still, even it just said "deletion" and we were to accept "deletion"=apoptosis, I don't think it would've been the correct choice. The problem in AIDS is not Th1 cells reactive for TB antigens jumping off a cliff like lemmings, but a general lack of availability of T-cells to interact with macrophages (acting as APCs) and becoming activated.

The answer is also not C. It represents a specific concept in TB infections: Why would some people with primary lung tuberculosis present with bacteremia and disseminated forms of TB? If there's a defect in macrophage phagocytosis (due to some gene polymorphisms I can't recall at the moment), then TB bacilli would continue to replicate in the macrophages, eventually escaping from cellular prison and wreak their havoc. Again, because of defective phagocytosis, macrophages wouldn't be able to activate naive T-cells to become Th1 cells.

As for the choice B, the problem in AIDS is not an inability of T-cells, but lack of T-cells. A form of SCID might cause such a maturation defect for T-cells.

 

[Ty Webb]

In my opinion, answer choice B is the least flawed of your options. Not A or C, due to the reasons given by Fuzuli.

Assuming that answer choice B was quoted verbatim (and the missing D and E choices were indeed incorrect), I would think answering the question would involve how liberally you're willing to interpret the term "inability" (i.e. if you're willing to extend the term inability to include any cause, such as decreased cell number).

"Clonal deletion" and "defect" are also much more specific terms, and not as open to abstract interpretation, which helps to eliminate A and C.