xelatex fail to show characters in rmarkdown

[Sam8]

hello,
when my text has symbols like ( alpha , Beta .. ) they don't appear when I knit pdf, but they do when the output is HTML or anything else. I tried using xelatex or Luatex as latex_engine but didn't work as well. does any one have the same issue ? this is what I used:

---
title: "FILE_NEW"
output:
  pdf_document:
   number_sections: yes
   latex_engine: xelatex
  word_document: default
  html_document:
    df_print: paged
toc: yes
toc-depth: 10

---


# Background

I really don't know why symbols don't appear when I knit pdf, this is an example:
IL-1β , IFN-γ

[technocrat]

I'd cut and paste it but Rmd doesn't place nicely with the discourse rendering engine

[Sam8]

not working, specially in case I copy and paste from another source which has a lot of these characters, I figured that's the only way to do so is changing the mainfont

[technocrat]

Many, but not all, \LaTeX characters work with the default latex-engine set in the global or projects options.

Untitled.pdf (74.6 KB)

[Sam8]

that's right, I tried both pdfLatex and xelatex engines and didn't work, but I think I need some greek font package to support Latex better, at least foe some special characters maybe.